phpBB Previous / Next topic Extension

Install

Download the latest release.
Unzip the downloaded release, and change the name of the folder to prevnext.
In the ext directory of your phpBB board, create a new directory named dmzx (if it does not already exist).
Copy the prevnext folder to phpBB/ext/dmzx/ (if done correctly, you'll have the main extension class at (your forum root)/ext/dmzx/prevnext/composer.json).
Navigate in the ACP to Customise -> Manage extensions.
Look for Previous / Next topic under the Disabled Extensions list, and click its Enable link.

Navigate in the ACP to Customise -> Extension Management -> Extensions.
Look for Previous / Next topic under the Enabled Extensions list, and click its Disable link.
To permanently uninstall, click Delete Data and then delete the /ext/dmzx/prevnext folder.