Manually edit Cantrip spell list?

My Artificer Tinker's Magic and Gnomish Lineage both give me Mending, so it appears as a prepared cantrip twice. My DM has graciously granted permission to swap one of the Mending cantrips for something else in my known cantrips list. That's easily done with pen & paper, but how can it be done in dndbeyond? 

It can't be done directly.

The best you can do is create a homebrew feat (or a homebrew copy of the Gnome species) that grants you the other spell you want.

Ahhh clever. I may go the Feat route

EDIT: adding a feat worked perfectly! I still have two Mendings in my list but that's easy to ignore.