Using relics and credits after completing single player

Hi. I've beat the game on single player and am now going back and getting all the achievements. I'm also getting relics that I missed (or didn't care about) the first time around. I can pick missions and their difficulty when I click on single player->campaign but I can' find a way to get back to the armory/lab to spend them. Is it possible?

Thanks!

 

Protoss relics/Zerg DNA samples only grant research points unless you've maxed out one or both research trees, in which case you will recieve 10,000 credits per research point. Replaying the missions doesn't grant you the credit reward twice; the only way to get more money from replaying a mission is if you missed an optional objective that gives credits as a reward.