If your headset is under warranty, contact customer support (live chat, email, WhatsApp) here.
If your headset is out of warranty but you live in USA, you could get the controller repaired by Fix my Oculus.
If you're game, you could try fixing the controller yourself:
Fix Quest 3 & 3S Controller Triggers in Minutes – Step-by-Step Repair Guide!
Otherwise, you can buy a new controller in the Meta Quest Store. Look under Meta Quest>Replacement parts. The link I gave you is for the Australian store, so make sure you select your correct region first.