Open Gmail and sign in to your account
Click the gear icon in the top right
Select See all settings
In the General tab, scroll to the Signature section
Click the signature you want to edit, or click Create new to make a new one
Update the signature text in the editor
Format it if needed using the toolbar
Scroll down and click Save Changes
To change the default signature for new emails or replies, use the Signature defaults dropdowns in the same section
On the Gmail mobile app, tap the menu icon, go to Settings, select your account, and update the signature if the option is available