How to loosen cable tension in disc brakes
I got the following from this website, and if you go to the website there are pictures as well.


  On the actual brake, loosen the caliper fixing bolts with an allen
  key, both top and bottom. Once you do that feed the cable through the
  brake caliper from the barrel adjuster at the top of the caliper
  through the fixing bolt at the bottom, and once you have achieved the
  correct tension then fix it down by tightening the bolt with an allen
  key. The next step is to squeeze the relevant lever whilst jiggling
  the caliper sideways in order to help it find its best seated
  position. Once you have done this, and keeping the relevant lever
  squeezed, you can now tighten the top and bottom caliper fixing bolts.
  After this, you will need to back up the inner dial (fixed pad) a
  couple of clicks before ending up with the brake pad in the middle of
  the caliper.