By using drag-and-drop in a double "group by" view of your Contacts.

1. Create a view that groups your contacts by Category and then by
"User Field 1". [BTW, you might consider giving the field a better
name.]
2. Change "User Field 1" to the value you wish for one item with a
given category.  You should then have two subgroups under that
category.
3. Drag the heading that had the old value of "User Field 1" (probably
"none") to the heading that has the new value.
4. Repeat 2 and 3 until you have changed all the values that you want
to change.