Release a zip tie intact by lifting its internal ratchet pawl with a thin tool, so you can reuse it.
Find the small square housing where the tail of the tie feeds through. Inside is a tiny angled tooth (the pawl) that bites into the strap's ridges and only lets it move one way. That tooth is what you'll lift.
Push the flat tip of a fingernail, a thumbtack, or a small flat-head jeweler's screwdriver into the head, right where the strap enters, on the same side as the ratchet teeth. Wedge it between the pawl and the strap to hold the tooth back.
With the tool keeping the tooth lifted, gently pull the strap backward out of the head. It should slide out in the reverse direction without the familiar click. Work slowly so you don't snap the tail.
Check the strap ridges for cracks or shaving from the pawl. If the teeth are chewed up, retire the tie — a compromised tie can slip under load. A clean strap can be reused several times.