Please note, V5 is currently not supported in on-premise environment.
Should you need or want to revert back to V4 of Page Designer, follow the below steps for successful uninstallation of V5 and re-installation of V4 in your Site Collection.

Step one-Navigate to Site Contents

Scroll or navigate to the LiveTiles Designer Manager and click, you will be directed to the Designer app 

Step two- Uninstall LiveTiles Design Pages

From here, scroll down to the Deploying Designer to Site section, the first item under that section will be: LiveTiles Design Pages
Click the uninstall next to it
You will notice the spinning circles as the item updates

Step three- Install Designer 

Now that LiveTiles Design Pages are uninstalled, you will have to Install Page Designer again. Scroll back up and click Install
After successful install, you will be prompted to Go to LiveTiles Design

When clicked, you will be redirected to the Landing Page in V4

Notes

You will still have the option to upgrade to V5 from the Landing page or in Design view from one of your pages

No content from existing pages, templates, sections, tiles, will be deleted by reverting to V4 or upgrading to V5.

Related Links 

Installing V5 Part 1
SharePoint Modern Framework (SPFx) FAQ
Custom Code in LiveTiles Page Designer


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