I have an 8657B with the high stability option. The frequency calibration is accessible via two holes in the add-on module on top, the one at the rear of the generator.
Read the manual. It's in there.
If you don't have the high stability oven, I can't help directly. In any case, there is a warmup time necessary before full stability is achieved. For mine, it's about 45 minutes, since while the oven is always plugged in, the oscillator isn't and so there is some drift.