Can't find the option for this anywhere... any help?

First you need to find the GOGmix. This is the difficult part, since GOG for unfathomable reasons decided to remove the "My GOGmixes" list when they last redesigned the site.

If you haven't bookmarked it, you can most likely find it by going to https://secure.gog.com/mix/[name_of_mix]
For example, the address to "GOG Games bundled with Soundtracks (N-O)", is https://secure.gog.com/mix/gog_games_bundled_with_soundtracks_no (spaces changed to underscore, all lowercase, and everything except a-z, 0-9 and _ removed).

Then it's easy, click the "Edit" button.

Ok thanks, yeah I forgot my mix name, I'll just start over. Weird that they broke the link to see the ones you've created.