How to revert to an earlier version on Lightroom Classic?

1. Open the Creative Cloud app that’s used to install/uninstall any Creative Cloud desktop application. 

2. In the Apps section, find Lightroom Classic and click the More Actions (…) menu. 

3. Click Uninstall. 

4. After it finishes uninstalling, in the same menu as in step 2, click Other Versions, and then click Install for the version you want. 

Creative Cloud Lightroom Classic uninstall.gif

Two limitations:

According to the published Adobe support policy, the Creative Cloud app provides previous versions only as far back as the last two major upgrades, which are currently versions 14 and 15. 

Any catalogs made with version 15 can’t be opened in version 14 or earlier. 

https://helpx.adobe.com/download-install/apps/download-install-apps/creative-cloud-apps/install-previous-versions-creative-cloud-apps.html