I bought a I watch 3 with cellular last June 2018 but the battery was running out so fast sometimes it only last 1/2 day , I don’t used it much no texting only received a few call. What do I have to do now?

make an appointment at genius bar and I'm pretty sure they will fix this problem! https://getsupport.apple.com/?caller=arshw&locale=en_US&storeID=R035&lat=41.889290&long=-87.623700

When you send it to apple store they will check it and will fix it or give you a new one and all for free .