On some bikes, it just won't fit.
There are a few things you can do. Some 2.125 tire's tread are wider than others, you might try a different tire. Also, the fender braces may be adjusted a little to give you more room. If the tire rubs on the fender where it sits in the frame, then it can't be adjusted. Where the braces bolt up, there may be some leeway to adjust them outward. Where the tire hits the brace, it might be possible to bend them outward slightly, just enough to clear the tire. You can also adjust the wheel inward at the same time to give some space, as long as the chain isn't too loose.