Well, Julie, the bad news is that you really can’t “repair” mildew damage. If the mold, which grows on damp surfaces, has become extensive, then it can weaken the fabric and possibly hasten the destruction of the tent in high winds.
On the other hand, if the mildew damage is light and generally cosmetic, then it is possible to kill the stuff and remove the stains. And, if proper care is taken to prevent a recurrence, your gear should be OK for some time longer. The tried-and-true formula, courtesy of the fine folks at Rainy Pass Repair in Seattle, who are now repairing a pack I blew out (warranty item, but I figured they’d make it better, not just make it good as new), goes like this:
First, kill the mold with a solution made from one half-cup of Lysol mixed in with a gallon of hot water. Then, rinse the material with a solution comprising a gallon of hot water mixed with one cup of lemon juice and one cup of salt. Allow the tent to dry, then soak the stains in another batch of the lemon/salt/water solution, which should get them out. Rinse thoroughly and allow to dry. (NEVER put your tent in a clothes dryer, as the heat will cause the waterproof coating to peel off, and then you’ll have real problems!)
Another option, albeit an expensive one, is to send the articles back to the manufacturers. They may be able to replace the most affected portions, either replacing entire fabric panels or fashioning patches to repair damaged areas. Make that Plan B, though, after you’ve tried the do-it-yourself fix, provided that the damage is not too serious.