How to change a forgotten iPad passcode?

It depends on which iPasOS version you are running.

If your iPad is updated to at least iPadOS 15.2 see this --> How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support

If however, your iPad is not updated to iPadOS 15.2 or higher, you will have to erase the iPad using your home computer with iTunes (Windows) or Finder (Mac) following the directions in this link --> If you forgot your iPad passcode - Apple Support

Note: If your iPad is disabled, you will have use the second link to erase it.

The procedure for iPhone is essentially just the same as it is for iPad:

If you forgot your iPhone passcode - Apple Support

The process will unavoidably erase all data and photos from the device - but as long as you have an existing iCloud or iTunes backup, nothing will be lost. The final stages of the process will restore your data from your chosen backup.