To edit an existing custom error page, follow the below steps.
Login to your Plesk panel.
Go to Websites & Domains tab.
Go to File Manager.
Go to error_docs folder.
Now you can select the file you want to edit.
Click OK when you are done.
To create a new custom error page, follow the below steps.
Create the error page in your system and upload it to File Manager.
Go to Websites & Domains.
Select Show Advanced Options.
Go to Virtual Directories.
Click on Error Documents tab.
Select the error document you wish to use and set the options like Type and Location.
Click OK to complete.