I just had to do the same thing for my daughter, bought a used LeapPad 3 off OfferUp. I had to download the LeapPad Connect software and install it on my computer. Once installed you can connect the used LeapPad to your computer and the connect software should see it. Select Parent Account from the top right corner and log in with your own Parental account, either new or existing. This new account will assume ownership of the tablet and then you can click on Settings in the top right of the Connect app and you should see the Reset button there.
Happy Holidays!
Greg