After New Version of WordPress 5.0 and Divi 3.18.2 launched all divi users going on testing websites.

Note:Before Upgrade site please backup of your website with database.

Take care about upgrade.

As suggested Nick.

Update Divi
Update plugins
Update WordPress

Most of no issue found after upgrade.

When you edit/add page or post you see when click on divi builder it is redirect to front end builder.

If you want to enable divi back end builder then Go to Divi->Theme Options ->Builder->Advanced

You can found below two options

If you want to enable classic editor (default word press editor) then Enable Classic Editor option only.

If you want to enable latest divi builder editor then Enable The Latest Divi Builder option only.

Once you set above option you will see backend builder editor.

For more information Go to Elegant Theme.

How to enable divi builder back end editor