This is something that has really annoyed me about the 6D. The answer is 'you can't '.
There has been some discussion on this before and Marsu42 has indicated that Magic Lantern have included it in their software, but that obviously means you must install ML to get the feature.
I see no reason whatsoever for the 'SET' button on the 6D not to be used as a 'back to centre point' button. In my post on this about six months ago I thought it must be a programming error, it's such a daft omission, but now it looks like it's deliberate. Ouch ! I use the toggle a lot on the 5DII and find the 8 way controller on the 6D wholly inferior.