At the top of the ring, there is a plastic section. You need to wiggle that out of the the ring. When it’s out, you should be able to twist it open in the middle and that’s where the batteries go. The on/off button is on one end of that too.
Is it this one? I don’t own it I’m afraid but was going to say the same as @Calie .
Normally you just remove essentially the bullet section and unscrew it to change the batteries. The button is on one end to activate it. If it is new, it will already have batteries in but they might have a bit of plastic guarding them and stopping them from working that you need to remove before the toy will activate.
The product video half way down the page shows how to activate it but unfortunately doesn’t display how to take the vibrator out to change the batteries.