Where is extension data stored?
Its a bad idea of upgrading as uninstall and install new version of extension. You can upgrade the extension either one of the following two ways:


If the extension is listed under CiviCRM extension page with auto-download as true then under CiviCRM extension page of your site you should be able to see an option for updating the extension(if option not available than make sure you have Resource URL set for extension correctly, CURL, WGET, ZIP extension installed on server).
You can also upgrade the extension manually by replacing the extension folder/files and then visit CiviCRM extension page in your site and click on refresh button. If the new extension requires db changes than you will be prompted with an option on top right of the browser to run db upgrade. Run the upgrade and its ready for use.


CiviCRM stores extension settings in civicrm_setting and civicrm_managed table. But some extension have their own install code to store data in various table, uninstalling the extension will lead to loss of such data as incase of mosaico extension. It is recommended to follow upgrade pattern for upgrading any extension.

Note: Always take backup of db and files before performing upgrade.

HTH

Pradeep