If you need to adjust the strap length you can do this by pulling the strap that has the small plastic wedge, this then releases the other strap so you can either make it bigger or smaller.

Lengthen the strap
1. Pull the upper end of the strap loop to lengthen it
2. Pull simultaneously on both ends of the strap loop 
   to fix the length
   
Shorten the strap
3. To tighten the strap, pull at the lower tail end of 
   the strap
4. Pull simultaneously on both ends of the strap loop 
   to fix the length