It is always a good idea to be running the latest version of WordPress and Plugins you may have installed on your site, as these have often been upgraded to fix known bugs, improve stability, and most importantly fix known security issues.
However, sometimes running the latest version can cause issues for your site due to incompatible themes and plugins, and at times like these, it is best to roll back your version of WordPress, so that you have a functional site, as you work out improvements for your theme or plugins, to make them compatible with future WordPress updates.
It is very important that you back-up your files and database before taking these steps so that if anything goes wrong, you can at least restore your site.
Step 1: You will need FTP software, such as Filezilla. Access your server via the FTP software and backup all your files in the root folder.
Step 2: Delete the ‘wp-admin’ folder and ‘wp-includes’ folder from the server
Step 3: Go to https://wordpress.org/download/release-archive/ and download the version of WordPress you would like to use.
Step 4: Once downloaded, unzip the files, then upload all the files and folders, except the wp-content folder using Filezilla or another FTP software.
Step 5: Visit your site admin login page where you will be notified that the WordPress Database needs to be upgraded, just click OK and after a few seconds, the login page will show, and everything should be working fine.