Easiest way is to type deselect in the search box of the Add-ons & Themes Manager, then select the first result for Deselect on Delete TB78.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

click hamburger menu in upper right of main screen then, ciick the addons option then, click the gear wheel and select the option to install from file

Be sure you are using the latest version from the Thunderbird Add-Ons site:

https://addons.thunderbird.net/en-US/thunderbird/addon/deselect-on-delete-tb78/

If you search for "Deselect on Delete" from "Add-ons and Themes" within Thunderbird, you can install it directly from there without manually downloading it. This has an advantage of the site letting you know if the add-on is incompatible with your Thunderbird version, or is perhaps untested or dangerous.

Otherwise, you can install an add-on from an already-downloaded file from (in Windows) menu item <Tools | Add-ons and Themes>, click on the little gear icon and choose "Install add-on from file" and browse to your file and choose it.