I had the exact same issue just today. Running Rstudio with administrator rights did not solve the issue.
With me the problem was that my Rprofile loaded "usethis", "devtools", and "testthat" on startup. One or more of these packages loads "rlang", and this prevented "rlang" from being updated.
I solved it like this:
file.edit(file.path("~", ".Rprofile"))
Comment out all the loaded packages.
Restart session
install.packages("rlang")
I hope this helps.