Most of the metro platforms in town have slopes instead of or as well as steps so you should be o.k. We usually travel from Peter's Square back to Chorlton.
I am travelling on the M/C Metro at about 1.30pm on Wednesday. Not having visited M/C this way before (usually drive) I am going to Chorlton and know I have to change lines at Deansgate or a couple of other points. Can any local tell me which is the best line crossing point for someone who is not good with steps (or or down)?