It will be rather simple as they are both located in the same terminal albeit different wings.
Follow the signs to transit, clear security, and then you will be back in the boarding area. Walk towards whatever gates United is using and voila. UA
have a customer service center located across from their Red Carpet Club where if you need to check-in or ask about your flights you can.
Time wise - the potential hold up will be security - if a bunch of flights all arrived at once with a few hundred connecting passengers the wait can be a bit, but otherwise should take not much longer than you being able to walk the length of the terminal complex.