I’m guessing this is related to your questions over in this other thread?
To find out your R version, look at the message you get whenever R starts up, or run sessionInfo(). If you are running a newer version of R than 3.4.4, I would not recommend downgrading unless you’re sure you know what you’re doing!
That said, older R binaries are available on CRAN. There are different pages for different platforms:
FYI, R releases (3.x) currently come out annually. This year’s version was R 3.5.