Changing 404 Page

I'm trying to change my 404 page to a custom page i've had made.

I've tried using 'on site template editing' as its the only place I see it referenced. Can it be done here or is there another way?

I'm running cart version 4.2.2 but will be upgrading to the latest release within a few weeks.

Design->Layouts->404

Thanks. Ill give this a try

Mike