The buckle holds the strap in place by a metal lever with teeth. Slip your finger under the lever and pull out.
The strap is just fabric and may easily be cut with a sharp knife. A good pair of scissors gives a clean cut and maximum control, however. For added life synthetic webbing may also be sealed off over a small flame; cotton may be sealed with a bead of glue like "Fray Check" from Prym-Dritz. Use due caution when cutting and/or sealing.
When the strap has been cut, replace it in the guide on the back of the buckle and snap the lever back in place. Be sure to push the strap far enough into the buckle for the lever teeth to get a good bite on the strap.