How to quit

I just recently got hired at this restaurant for a full time managing position but unfortunately I’m not enjoying myself as much as I thought, the work load is extremely heavy on my body and I’m going home in a lot of pain when we are opened, I’m in Melbourne and currently in lockdown so I don’t even have my full time position. I’ve gotten offered a job with better hours, an office job so I’m not doing heavy work, and better pay… I feel extremely torn as I’m loyal to the restaurant but I feel they will never value me. How do I tell my boss I’ve found something better? This all happened so quickly