Honestly, the best way to lift an urban bench off the ground for cleaning underneath depends on whether it’s bolted down or freestanding. If it’s a typical public bench with surface-mounted legs, you don't need to fully remove it. Here's the approach I've used and seen work well.
First, inspect the bench. Check how it's attached to the pavement. If you see bolts, grab a socket wrench or hex key and remove them. If it's freestanding, you can skip that step. Enlist a helper—urban benches are often heavier than they look, especially metal and concrete-slat models.
For cleaning underneath without dragging the bench, use a pry bar and a couple of wood blocks. Wedge the pry bar under one side's frame, lift just enough to slide a block under each leg on that side, then repeat on the other side. This raises the bench a few inches and keeps it stable while you sweep, scrape, or rinse underneath. If you have a car jack and a length of wood, you can also jack up the center crossbar gently, but never put your hands under an unsupported bench.
Avoid flipping the bench onto its back; that can bend armrests, chip concrete, or damage the finish. Once it's raised and supported, use a long-handled broom, scraper, or leaf blower to clear debris. For stuck-on grime, a pressure washer on a low setting works well, but keep the spray away from bearings or moving parts if the bench has them.
After cleaning, lower it slowly. Remove the blocks one side at a time and check that all bolts are tightened back down if you removed them. Give the area a final sweep. That's it—lift a few inches, support it properly, and never work underneath unless it's securely blocked.