There are two ways of uninstalling Airmail and setting it up again. These are described below.
Simple Uninstall and Re-Installing from App Store
To accomplish this method, please move the app to trash from applications folder as shown in the video below.
Then move to App Store and re-install Airmail. If App Store asks to pay again, make sure to log out and log in back in the App Store to avoid such situation.
Using this method, you will not loose any of the preferences from Airmail.
Deleting the Airmail Libraries and Re-Installing
Please note that this method will remove all the preferences and data of Airmail from your device and you will have to setup Airmail again from scratch. To reduce the time taken for setting up accounts, it is advisable to save your account settings and preferences to iCloud. Please also see here, how to back up Airmail and Airmail data.
To accomplish this method please follow the steps as described below:
1. Move the Airmail app to trash from Applications as shown in the video above.
2. Open Terminal: (⌘+Space Bar then type Terminal) and copy paste the following command and hit enter.
rm -r ~/Library/Containers/it.bloop.airmail2
Please refer to the picture below:
3. Then re-install Airmail from App Store and open to proceed further.