If you remember the passcode, go to Settings > Face ID & Passcode or Touch ID & Passcode > Change Passcode
If you forgot the passcode, put the iPad into recovery mode
Connect the iPad to a Mac or Windows PC with Finder or iTunes
Open Finder on Mac or iTunes on Windows or older Mac
Select the iPad when it appears
Choose Restore when prompted
Wait for the restore to finish
Set up the iPad again
If asked, sign in with the Apple ID used on the iPad
If Find My iPad is enabled, use iCloud.com/find to erase the iPad remotely
After erasing, set up the iPad and create a new passcode