Nothing other than being as generally healthy as you can. Eat well, drink lots of water, get plenty of sleep. If you are going to get sick, nothing will truly prevent it. Once you do have symptoms, there will be things you can take to relieve them (unstuff your nose, relieve aches and pains etc).
That said, although I totally understand how unfortunate it seems... it won't be the end of the world. If you perform and don't do your best, or if you decide to sit it out entirely. Colds happen. Singers get sore throats, lose their voices. It sucks, but such is life. You'll get better and will sing again.
I only say this because I know I personally tend to overreact to things, particularly those important to me, so if I were in your situation I would probably be having a panic attack and thinking of all the ways it will change my opportunities in the future and all that stuff. So, if that's at all what you're feeling, don't lol. It will alllll be okay. Plus, stressing mentally can stress out your body, which could exacerbate the cold. And we don't want that now do we?
So yeah. Get plenty of sleep. Lots of water, healthy food. And relax!
Best wishes
