Depends on which nostr client you're using. I am using coracle.social from a laptop using Firefox. In Coracle, when I go to zap someone, if I want to send a different amount, the trick is to long-click the ⚡ button to get a prompt to come up where it asks you the # of sats you want to zap.

If you’re using Yakihonne you can change it by long pressing the zap ⚡️ button