Can I decarboxylate my marijuana after I have already made a tincture with it?

"I used non-decarboxylated marijuana to make my tincture."

Sorry you may be out of luck here. You can try to heat your tinture in a container, though decarboxyliation occurs at 220 degrees F so that is not so simple and probably a pretty dangerous idea. You could evaporate the tincture solution, leaving your THC behind. Then set that in the oven at 220 for 30 minutes, and reintroduce a new tincture solution.

These are just some ideas. I really don't think they will work that well. You will most likely have to start over.