Once you selected it you are locked in how every much you contributed until the period is over. 😅

I think you can make changes/adjustments on bean stock (reduced price Starbucks stocks) once a quarter and I think it lasts for a couple weeks. Check on the Fidelity website. I think it'll give you the dates in advance, but maybe once a week till you see it. Not sure what screen it'll be on.
401k (if you mean the retirement fund stocks) can be adjusted at will.