forgotten passcode

The only mechanism to reset the screen lock passcode is to restore the device, erasing it so a new passcode can be set. There is no alternative mechanism to reset the screen lock passcode. Once the passcode has been reset, you can restore your data from an iCloud or iTunes backup file. But without the backup already in place, data loss is inevitable.

See -> If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

if the phone is not disabled yet.. and you've backed up to a Mac before, you might be able to plug in your phone and make a backup so long as the 'trust' has already been established.

I don't back up to iTunes/Windows, so I'm not sure if it's the same.

rhea125 wrote:

how do you restore the device without unlocking it

You don’t unlock it. If you’ve read the instructions in the link I provided you will see that you put the device in recovery mode and then restore it.