So today I gave my sister a bootable San Disk USB with Ubuntu on it. However the storage space on it, after being used, went down. When I first created the USB drive, I accidentally only partitioned the drive for about half of it to be Ubuntu (just me badly misunderstanding). I looked it up, and found the diskpart command for command prompt, however the instructions on it wouldn't work with my bootable USB, and I couldn't find any other tutorials that gave results. How could I remove this unused partition just taking up space and add the space from that to Ubuntu?
EDIT: forgot to mention, but I'm on Windows 10.