It's a bit fiddly to change watch straps on a F91W. You want to put spring bars in there, which is what watch straps attach to, and you'll have to use a bit of

It's a bit fiddly to change watch straps on a F91W. You want to put spring bars in there, which is what watch straps attach to, and you'll have to use a bit of force. A sharp knife helps. There are plenty of almost-similar models, with the same low prices and some have standard lugs. The A158W is one of them. Then buy any strap you want, leather or textile are nice and cheap. You'll want 18mm wide bars and straps.