Get a strap kit for the SL500 watch (available on the Decathlon website) and a pair of pliers. The kit contains the two strap pieces (top and bottom) as well as two new pins. - Using one of the pins, push the old pins out of the strap. To do this you can push it against a table, for example.- With the pliers, pull out the pins.- You can now remove this part of the strap.- Replace the old strap with the new one, while choosing the correct strap (top or bottom).- Replace the pin by pushing it all the way in. Make sure the pin does not stick out by running your finger along the surface.- Do the same thing for the other part of the strap if necessary. You can also go to a Decathlon store or to a watchmaker.