The physical SIM tray can be awkward to eject. Using the SIM Tool supplied with your iPad, or a partially unfolded paperclip, place the tip in the tiny hole of the SIM tray. Do not insert the tool into any other aperture as you will cause damage.
Remove or switch the SIM card in your iPad - Apple Support
While keeping your fingers clear of the SIM tray, apply inward pressure to just the SIM Tool; you may have to press quite hard. The SIM tray, with the SIM Card, should pop-out.
If you are unable to remove the SIM tray from your iPad, you would be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to assist:
Genius Bar Reservation and Apple Support Options - Apple