Hello,
It looks like you have the original Series 370 window that was designed before takeout clips were used. The hex screw is used as a take-out clip. First, you'll need to remove the "L" bracket above the hex screw on both sides of the window. Then, you would raise the sash above the hex screw to release the balance from the sash. Please one hand on top and one on the bottom of the sash and push the it to one side to carefully swing the sash out.
To remove the balance, remove the hex screw and slide the balance up to the cut-out at the top of the window track so the top balance guide wings can be removed from the track. To install the new balance, fit the wings of the balance guide into the track cut-out, bring the balance down, and install the bottom balance hook into the square hole in the middle of the track. Next, hold the balance in place while someone installs the hex screw back into the track. Slowly release the balance to meet the hex screw. Install the sash by sliding one side just above the bottom shoe, swing the other side of the sash in, and bring the sash down to connect to the balances. Install the top "L" brackets for the window stop. See the related media guides shown on this page to assist you. Thanks for posting!
