If you squeeze the brake lever and look inside using a small flashlight, you should see an Allen head bolt at the bottom. That tightens the steel strap around your handlebars. Loosen it (probably a 5mm and you will probably need a 6"/15mm crescent wrench or the like to turn the Allen wrench), turn the brake lever and re-tighten.
I've never owned brifters so I'm guessing they are the same as the brake levers I've been dealing with for 50 years. If I"m wrong 1) you won't see that Allen head - I have seen regular hex heads and even screwdriver slots - and 2) another poster will jump in soon and correct me.
Edit: if you do see the Allen head, release the brake cable; either with the lever on the caliper or by undoing the cable nut. That will make your job of getting the wrench in and properly on much easier.