Microsoft does not appear to offer any repair services for the Hololens. The thin adjustment strap can easily be broken if it is forced, and there is little that can be done to repair it if this happens. The only solution I was able to devise was to replace the broken adjustment strap altogether. I cut the strap off near where it joins the body of the "halo", leaving 15mm of strap attached to the "halo" for a lap joint. I marked and drilled two holes in the remaining strap to match the holes in the replacement part, inserted two rivets made from short sections of ABS filament (I heated the end with a lighter and squashed it into a mushroom shape), and glued and clamped the parts together with Gorilla Glue. Once the glue had cured, I heated and flattened the other end of my ABS rivets to provide a very secure attachment. I lost 15mm of adjustment, but the part works and the Hololens can once again be secured to my head. This repair requires the disassembly of the adjuster, which requires only a small star drive bit.