That's brilliant! So what we are going to do is put your phone into what is called DFU mode, which will allow you to update your phone to the latest version of iOS without it actually booting up. I say this because you are currently on iOS 12, an outdated version.
Step 1: Connect your iPhone to your PC with a cable.
Step 2: Press and release the Volume Up button, press and release the Volume Down button, and press and hold the Side button for at least 10 seconds.
Step 3: While still holding the Side button, press and hold the Volume Down button for 5 seconds, and then release the Side button while continuing to hold the Volume Down button for another 10 seconds.
Step 4: Launch iTunes on your PC, and you should see a message stating that “iTunes has detected an iPhone in recovery mode. You must restore this iPhone before it can be used with iTunes.” or something similar of that nature.
If this doesn't come up, select your phone in the sidebar. I don't have the best experience with iTunes for Windows, so I apologise for not knowing exactly where, but somewhere on the screen should be a button which says 'Update iPhone'. Make sure that you do not press Restore iPhone, as this will wipe your data.
It should now update your phone, giving you instructions on the computer.
I hope this was helpful, if you have any questions let me know.
Cheers,
Jack