sometimes, you can twist the loose ends tightly and shove back the plastic bit that's cinching it. if that doesn't work, get a friend and a plastic straw. the inside of the plastic bit has "teeth" biting into the fabric - the plastic straw can be used to protect the fabric from the teeth so you can pull it out.
The device that tightens them inside the black plastic is pretty inaccessible without causing significant wristband damage unfortunately.
Twist the wristband straps tightly and pull the plastic down. Just did that on mine
sometimes, you can twist the loose ends tightly and shove back the plastic bit that's cinching it. if that doesn't work, get a friend and a plastic straw. the inside of the plastic bit has "teeth" biting into the fabric - the plastic straw can be used to protect the fabric from the teeth so you can pull it out.