The properties of each node has a display order. Just adjust that.
So if you have a category called CATEGORY 1 and a forum called FORUM A and FORUM C and you now want to add a FORUM B between them, you would change the order of FORUM A to 1, the order of FORUM C to 3 and the order of FORUM B to 2.
A good convention is to use large spaces between numbers.
So FORUM A could be 100, FORUM B could be 200, and FORUM C could be 300. Then, in theory, you could insert 100 more forums in between FORUM A and FORUM B without having to change their orders as well.
Imagine if you wanted to insert a new forum in between FORUM A and FORUM B. You'd have to change FORUM B to order 3, FORUM C to order 4, then insert FORUM X to order 2...