WP User Manager is a complex plugin that extensively interfaces with your site’s database. From time to time, modifications to the database structure itself need to happen, whether to increase performance or to resolve issues. When that is the case, you will see a notice within the administration panel of your website asking you to upgrade the database.
If you are experiencing troubles with the database update, in general, there are three types of issues that will conflict with the database updates: Caching, Security Plugins, and Admin-Ajax issues.
If you have any plugin or server side caching enabled, please flush the cache and temporarily disable the caching system. This resolves DB updates for a significant percentage of users.
Security plugins ( like Wordfence ) many times will prevent changes to the database if they think these changes shouldn’t happen. Temporarily disable any security plugin installed on the site and proceed with the database update.
If the admin-ajax.php file is blocked, database updates will not function. Please disable any security plugins and make sure your host has not restricted access to admin ajax.