The whole point of a resident AV is to protect you. If some malware were capable of disabling your resident AV in such simple manner (as you want, but manually), then you wouldn't be protected.
As mentioned, there are non-resident security tools too.
As for a more technical answer, for you to disable avast, you would need to disable the defense module, the driver, the service and the shields of avast. I can't see the point of doing it, but that's up to you.
Alternatively, you could do whatever you want under Windows Safe Mode, without touching avast. Then avast would work again in Windows Normal Mode.