How do I get off erasing screen?

Right, so I bought my son the Hudl, from an exchange shop, was setting it up and noticed it was running very slow.

I hard reset it and when it gave me the options to factory reset etc i accidentally pressed the “update using ADB’.

I am now stuck on an erasing screen, if I press the power button and the volume up button I then get the “send the package you want to apply to the device with ‘adb sideload’” I have no idea what I’m meant to press on this, if I press the volume up and power it goes back to the erasing screen and will stay on that for ages.

I just need to get back to the hard reset page so that I can press what I need to press.

Can anybody help?