What if you use 20 amp instead of 15 amp fuse with the same 250 volts in microwave?

In most cases the size of a fuse is not critical and so changing from 15 to 20 amp will not effect anything -

BUT the function of a fuse is to destroy itself before some more expensive component gets destroyed by too high a current flow. As you raise the current rating of your fuses you increase the chances of negating their function,