Look at the bottom of each side where the string attaches to a small round "adjustable disk". It probably has a Phillips head screw in the center. If you loosen that screw and unwind the disk about a quarter turn, it will take tension off the shades. Just don't let it unwrap all the way or you will be rewinding it and that isn't fun. Loosen it only a little at a time and don't forget to tighten the screw back down when you finish the adjustment.
2013 Silverado 2500 HD Duramax/Allison Tuned
Aviator Wright Flyer #1919
"First In, Last Out". 11B4Y
DAV, MOPH, VFW, 5/7 Cav,1/8 Cav Assoc.
11th ABN Pathfinder Co. N7MIP