There’s no magic trick to dismounting these tires. It's just a conventional tire bead and seat. The rims can’t be split. They can be messy to dismount because the rims are deep and tires are usually old and dry. The small 6" size doesn't give you a lot of flexibility around the bead. Use lots of bead lube and patience when dismounting and mounting. I’ve used a HF small tire changer on these but needed to make an extension to work with these deep wheels. Often the hard part is getting the wheel off the axle if it's never been off in its life. It usually takes many soakings with liquid penetrant, application of heat or a slide hammer… or some combination of all.
If it’s holding air, why dismount it? These tires haven't been in print since the seventies, so if the bead gets wrecked, you're done.