Disconnect the strap from the output jack strap button. Grasp the loop side of the Kevlar where it wraps around the leather strap and start pushing the leather strap through the Kevlar loop. Pass the entire strap through the loop – the buckle may be tight but it will pass through. Once the entire strap is pulled through the loop it will be free from the instrument.
Reattach by passing the Kevlar loop through the hole in the end of the leather strap, and then pull the entire strap through the loop. Should the Kevlar loop ever need to be replaced or removed, it can be detached from the instrument by loosening the strings and unbolting the neck.
To shorten the strap, remove it as above and recut the length and hole for the Kevlar loop.