It would be a lot easier if you would let us know if you have baggage, and the final destination.

It is normal procedure for UA to rebook if you miss a connecting flight that is on the same ticket. Check and see if there are later UA flights to your final destination.

It takes about 10 minutes to walk to immigration from the gate. Walk fast. Last time I was in ORD a week before Easter, there was only one person in immigration the first 25 minutes, and it took forever to get through. This is not normal, but stuff happens.

Once you pick up your luggage (priority works about 50% of the times for me) you have to que up at the exit to give customs the white customs form. If there are two custom agents, the line queues from the left and right. It can sometimes be an advantage circling around to use the guy on the left. There is a 10% chance of getting diverted to extra luggage check, but this normally is just putting your luggage through a scanner. They expect you to put all you luggage on the belt. It takes about 5 minutes. Taking no items (that you are leaving) into the States will help avoiding this extra check, but is no certainty.

If you exit through the normal exit (for this flight) on the right (exit B) and if you you have 45 minutes or more, drop your luggage off on the belt on the right, just before you exit into the public area. Hurry to the stairs, escalator to the train. You still have to get from terminal 5 to terminal 1 (there is no terminal 4), and then through security again. It can also take an extra 10 minutes if your connecting flight leaves from Concourse C and as opposed to Concourse B.

If you're late, use the checkin counter to the left of the doors before you go out to the public area. If you are in doubt, there are normally UA people you can ask for help at the transfer luggage belt.