That's going to depend on the client you are using. It is best to remember when asking questions about how to do something on Nostr that there are a wide variety of clients that all have different ways of doing the same thing. So, always include the client you are using when asking your question to get the best quality answers quickly.

Some clients allow you to long-press on the zap icon to change your default settings. Other clients will require you to navigate into your settings options and find the zap settings there.

Some popular clients:

Amethyst - Long press the zap icon.

Primal - Go to Settings>Zaps

Yakihonne - Go to Settings>Wallets and scroll down to the Default Zap Amount setting.

Coracle - Go to Settings>App Settings and Default Zap Amount will be right at the top.

Jumble - Go to Settings>Wallet and if you have a wallet connected you can change the default zap amount.

Nostrudel - Go to Settings>Lightning and you can set various zap amount options separated by commas.

As you can see, they are ALL slightly different. So which are you asking about?