I'll reframe the answer I wrote for Python, for R. It's the same process:
The Changes.md
file is the easiest place to look for links to where you can download every version. Just search for the version you want (e.g. "3.30.0.1") and you will see the URL there.
Click on the link and it will bring you to the download page for that version (e.g. 3.30.0.1) and you can click on the "Install in R" tab which will show some code like this that you can copy/paste into your terminal:
# The following two commands remove any previously installed H2O packages for R.
if ("package:h2o" %in% search()) { detach("package:h2o", unload = TRUE) }
if ("h2o" %in% rownames(installed.packages())) { remove.packages("h2o") }
# Now we download, install and initialize the H2O package for R.
install.packages("h2o", type = "source", repos = "http://h2o-release.s3.amazonaws.com/h2o/rel-zahradnik/1/R")
The URLs are "predictable" but you have to know the name of the release as well as the version number to correctly guess the URL.