There's no "reset" for EX1 pedals, unfortunately. The only thing you could probably try is to use a different 1/4" patch cable to see if the behavior changes.
Another thing you could try would be to take your M13 to the nearest Line 6 Dealer in your area. They should have other expression pedals in stock; see if you can't ask a salesperson to let you try a few out with your M13.
Do those pedals behave differently? If not... then the issue is happening internally to the M13. Take your M13 to the nearest Line 6 Service Center. If yes, then your EX1 pedal is faulty (assuming you've tried a different 1/4" cable already).
Hope this helps.
[NOTE: this may apply if you use a pedal board]
I have two expression pedals with my Line 6 M13. They are mounted with the unit on a pedal board and plugged in all the time. What happened to me was that the sweep would not engage to the full 100%. What I found is that I have to unplug the expression pedal cables from the M13 prior to turning it on then plug them back in once it is turned on. That fixed it for now. My firmware version is 2.04.
I just picked up an EX-1 today for my HD-500. When I got home, I saw that the plastic casing for one of the plugs on the included patch cord was cracked and it fell to bits when I plugged it into the HD-500. I tried it anyway. At first I thought it was working, but then I saw the same thing as is being reported here. It jumps from 0-20% then has a smoother (more expected) linear increase after 20%. This makes it unusable as a volume pedal.
I tried a couple of other short (10") patch cables I have, and the problem persists so I know it's not the cable. I suppose 3 or 4 reports of the problem don't prove a trend but it's a bit strange that all the reports claim a problem from 0-20%.
I think the shop has one other one left so I'll try exchanging it (and I'll test out the new one at the store). If it doesn' work, can anyone recommend an alternate expression pedal?