You have to use a partition utility like GParted.
If you are using APFS as filesystem GParted cannot modify it. You'll have to find a tool able to do so, and do the resize in parts, first the physycal APFS partition and then the Windows NTFS. If there's no software which automatically does it it involves a VERY HIGH RISK of data loss. Keep this in mind and backup your data (or whole disk) before.
If you want to try the "easy way" first follow this guide about resizing bootcamp using minitool partition wizard. It will only work properly if the NTFS partition of the Windows installation is a proper NTFS partition. Keep in mind that BEFORE doing this you'll have to shrink your mac partition using mac's Disk Utility. It will not allow you to make the bootcamp bigger, but it will make room for you to enlarge it with MiniTool (or any other tool capable of working NTFS under the partition layout your HDD has). The full procedure is explained here:
Yesterday expanded my bootcamp partition from 50 up to 100 Gb using Mini Tool Partition Wizard Home Edition, a Freeware Windows Program after seeing this video: http://www.youtube.com/watch?v=WeKeawqxUa0
Note that I have OSX Mavericks, Steps are:
1) backup all your files (optional but strongly reccomended).
2) Using OSX Disk Utility resize MacintoshHD partition leaving unallocated space as much as you need to add to bootcamp partition.
3) Set Bootcamp as Boot Disk and restart the mac.
4) in Bootcamp with Windows running launch Mini Tool Partition and select the bootcamp partition.
5) Choose "expand" option in order to expand the partition using all the unallocated space.
6) The program prompt to you to restart the mac for apply the task, cause is not possible while the partition is mounted and windows running.
7) Restart the mac and then automatically Mini Tool Partition will do the job in a "graphic msdos like" form.
Now the long way around:
First of all, before doing anything MAKE A BACKUP OF YOUR HARD DISK. For zero risks use dd (diskdupe) to backup every single bit of the harddisk into a disk image. You can check how to do so here. If you dont want to risk to reinstall everything from scratch make sure your backup contains the partition table and the partitions, or that you are totally able to restore that backup, even if you bought a new hard disk. This is a critical step if you're not confortable with partition modification (its recomended even if you are). Whenever you touch a partition you are risking your data, and even though most times there will be a way to recover your info even if you screw it up without a backup, there are chances you'll overwrite important data and loose it forever. The risk depends on the operation performed and resizing is a particularly risky operation!. If you are curious about what partitioning is about, you can check this beginner explanation to get some insights of what you are actually doing.
Then you have three chances:
- Install a partition edition software on your Mac. I'm not a mac user, so I can only give you one example, Disk Utility, which I believe it is included in mac distros. You can check this discussion to find more examples. All I could find on google was comercial software, but as I said, I'm not a mac expert, so there may be a GParted port around or something like that.
- Boot using a liveCD/USB like SystemRescueCD or GParted Live and use GParted which is installed in the live distribution. There are many CDs to do this, like Partition Wizard, Partition Manager, Partition Magic, etc. In order to use thid method you will check how to boot using a CD on mac, which is a different topic.
- Do it from your Windows OS instead from MAC following a Windows tutorial. There are plenty around, like this one.
Personally I allways use liveCDs to modify partitions, as they are much safer. Why is this? Because if you resize a partition you are using (from whithin the native OS) your system has to reboot and enter in some short of "special boot envinronment" (its not that, but just to simplify) and resize the disks from there. Those aditional steps add many failure points to an operation which is enough risky by itself. If you can, allways resize OS partitions from liveCDs.
And remember, make backups! This is a critical step, and deserves to be repeated over and over my post. MAKE BACKUPS!
Also, a link about resizing APFS. I'm sure you can shrink (or increase) the virtual windows partition first and then the actual partition. I don't know if the guy who said it is impossible took all his knoweldge from youtube tutorials or he's an actual computer engineer, but as a compute engineer i'm telling you there's no such thing as impossible, specially if apple provides a tool to resize. However, take into consideration what he said and DO A BACKUP!
More questions about APFS resizing here