It depends on how you want to go about it - ideally you'd do both at the same time, but if you have good life left on the tires and don't want to wait on wheels, then you can do wheels first. Doesn't really matter what order - what matters is do they match. Meaning: are you getting larger diameter wheels? If so, obviously your tires won't work. Are you keeping the diameter the same but getting wider wheels? Then make sure your current tire will stretch/fit. As long as they will, you can do wheels first.
When I bought my DD new, I changed the wheels immediately. But the stock tires were brand new. I ended up keeping the diameter the same but got wider wheels. So I just had the stock tires mounted on the new wheels. They were a little stretched but the wheel width was in the acceptable range for the tires. So I drove them until it was time for new tires, and then got wider tires.
Hope that helps!