How to add or change signature in Gmail - Google

 Add or change a signature from Gmail Account (Computer)

You can put up to 10,000 characters in your signature.

• Login  Gmail from computer / laptop
• In the top right, click Settings  

• Click on See all settings.
• Scroll page and find Signature Section
• In the "Signature" section, add your signature text in the box. If you want, you can format your message by adding an image or changing the text style.
• Save now.

How to add or change signature in Gmail (Android) ?

Add or change a signature

Set up a signature that shows up only for emails you send from the Gmail app.

1. Open the Gmail app in smartphone.
2. In the top left, tap Menu.
3. Scroll to the bottom, then tap Settings.
4. Choose the Google Account where you want to add a signature.
5. Tap Mobile Signature.
6. Enter the text for your signature.
7. Tap OK.
8. Now save signature