I have just set up Chrome with my Google Account, and an application-specific password has been assigned to me... this password is long, and I need to enter it quite often...
Does anyone know how to change it?
You cannot change application passwords, they can be only generated and revoked.
However, you should not need to enter it more than once – at least not within Chrome itself; the program should remember it for as long as necessary. If it does not, you are likely using a very nonstandard configuration.