Changing the cancellation period can be done in the settings menu of Gmail using a desktop or laptop, not the app. After you open Gmail in your web browser of choice, click the “settings” icon in the top right corner above your email list. Then click, “settings” and look at the “general” tab of your Gmail settings, where you will see an option for “undo send” with the default 5 second cancellation period. This can be changed to a period of 10, 20, and 30 seconds from the drop-down menu.