1. Check app/Services/HousekeepingService.php and index.php to delete unwanted files from previous version while upgrade and update upgrade.php accordingly.
2. PHP REQUIREMENT LINK: https://webtrees.net/install/requirements/
3. Download webtrees-x.y.z.zip package from github and not source code as webtrees-x.y.z.zip contains language files.(confirmed vendor)
4. Delete entries of SITE_UUID,LATEST_WT_VERSION and LATEST_WT_VERSION_TIMESTAMP from site_setting and log table, created when accessed.