To add/edit or remove a custom redirect, please open Permalink Manager URI editor and click on “Add Extra Redirects”. After that, the list of custom redirects (created when the permalinks were changed) will be displayed. You can set-up new redirects and edit or remove the old ones.
How to automatically set-up redirects for old permalinks?
If you would like to automatically create a custom redirect for old permalinks after they are changed to avoid 404 errors you need to enable “Auto-create “Extra Redirects” for old permalinks” option in Permalink Manager settings:
How to display the list of all custom redirects created with Permalink Manager?
The custom redirects list can be displayed in “Debug” section in Permalink Manager admin section: