Success finally!!! I have struggled with this strap longer than I want to admit. I tried suggestions from both
@frenziedhandbag and
@SmokieDragon. I got so frustrated that the strap didn't budge so I was desperate and took out a screw driver and used it to make the hole area stretch a bit as I pulled an tugged at it. I did this to the hole closest to the end of the strap. Once it released, I think I did a tug of a combination of both your suggestions on the next hole and it released easily. I am so much happier with this bag now!
Be careful if you ever use a screw driver to do this as it could slip and stab yourself. I was so determined to adjust the strap that I had to take the chance.
Thank you to both of you for your tips and encouragement!