If you took a backup ( repository database, Certificate and Apps) from old Qlik Sense version, you can install/restore the old Qlik Sense version.
Reboot Windows OS
Stop all Qlik Sense services on services.msc
Delete or rename old Qlik Sense folder (Software folder that Qlik sense installed):C:\ProgramData\Qlik
Uninstall Qlik Sense:
Go to "Control Panel >> All Control Panel Items >> Programs and Features" and uninstall current Qlik Sense with patch version
Ensure the uninstallation of old version on "services.msc" and "Control Panel >> All Control Panel Items >> Programs and Features"
Install the same version of Qlik Sense with Repository backup.
Restore Repository database
Please refer to step of"1-2. Restoring on a different machine (PostgreSQL)" on this article How To Take Backup And Restore Qlik Sense June 2017 or higher You should follow the step of"1-2. Restoring on a different machine (PostgreSQL)" as Qlik Sense has been reinstalled.
*** You don't need to change the Shared folder.
Solution #2. The new installation process
If you don't have a backup (Repository database), you should install a new version of Qlik Sense and then import Apps and certificate, and configure token and UDC for users access.
Take a backup of Apps
Take a backup of the certificates
Reboot Windows OS and stop all Qlik Sense services on services.msc
Check the System requirement for your intended installation
Uninstall the old Qlik Sense installation Qlik sense (e.g February 2018 )
Delete or rename the old Qlik Sense folder (Software folder that Qlik Sense installed: C:\ProgramData\Qlik
Install the new Qlik Sense April version with a different Shared folder location (Not the same folder e.g \\Hostname\QlikDataShare\Apps )
You should create a different shared app folder or empty old Shared folder after backup during the installation