When you search for flights, click on the UPGRADE drop-down menu and choose either "miles Y/B" or "miles M/H". These are the only fare classes that can be upgraded with miles right now.

But, UA is implementing a new miles/co-pay scheme starting in early January which will allow all fare-classes to upgrade for a set amount of miles and dollars.

Check out this page: http://www.united.com/page/article/1,,53079,00.html