Updating stan_version() from 2.21.0 to 2.31

Hello everyone,

I am trying to fit some models following this chapter from the Stan functions reference manual 18.2 Multinomial distribution, logit parameterization | Stan Functions Reference

To my surprise, I could not find the `multinomial_logit’ while I was trying to write the stan code for my model and using that function gave an error. Then I realized that my stan_version() is 2.21.0 whereas this documentation refers to 2.31. I looked into web as hard as I could, but I could not find a way to update my stan version to 2.31. Can anyone please help me with this?

R version 4.2.1 (2022-06-23)
Platform: aarch64-apple-darwin20 (64-bit)
Running under: macOS Ventura 13.0.1

Thanks for your kind help!