I get the light cantrip from being an Aasimar and I get light cantripfrom being a Celestial Warlock in our Curse of Strahd campaign.

The DM has said I can replace one of these cantrips  - and I want to take mindsliver.

However, on my sheet under manage spells I cannot remove the rendunacy (as they are added from my race and class) nor can appear to learn Mind Sliver.

So how can I replace the redundant cantrip? 

Make a homebrewed copy of either your race or subclass and change it in the editor. That’s the only way. You could also just deal with the double instance of Light and simply add the new cantrip via a homebrewed feat.

I hope that helps.

Thanks I was hoping there would be a way I could just y'know add the spell without having to muck around recreating a race/class for a straight cantrip swap. *sigh*

Alright, thanks I'll have a ponder. 

It really isn’t that difficult. If you use the original subclass as the template for your remake you would just need to copy/paste the description in, change the name to something recognizable, and swap that one cantrip. Everything else would come preloaded from the template. Take you less than 10 minutes, less than 5 if you know what you’re doing.