If you're just canceling, then the Cancel function on united.com available when you pull up the PNR will undo the check-in automatically if it is necessary. Using the Change function also cancels check-in if necessary. Under most circumstances (assuming united.com is working properly) you only need to manually undo check-in if you need undo check-in but want to keep the reservation in-tact (e.g. so a travel agent can modify it without losing the existing segments). If that is the case, if you re-start check-in on united.com there is a tiny link at the top for canceling check-in:


(Note that I haven't had to do this in a while--I stole this image from some United documentation. It's possible it's slightly different now.)