The only thing an Administrator can do is access the Admin CP. It just so happens that administrators often have other permissions assigned to them effectively giving them full control, yet, at the same time, it is possible to create an Administrator that only has access to change Users in the Admin CP but doesn't give them any extra permissions on the forum.
So no, you can't edit whatever you want.
In terms of the RM, only the resource owner can post a resource update. Whether you can edit someone's post or not has nothing to do with it, that's a completely separate permission for a different purpose. If you are not the owner/author of the resource either they will need to post the update, or you will need to reassign the resource to you before you can do it yourself.