From what I have seen on this feature request, it is already available (this was the pull).
RStudio calls this Global Replace:
RStudio has long had a Find in Files feature, which makes it possible
to easily locate text in your project. If you’re not familiar with
this feature, try it out: press Ctrl+Shift+F (MacOS: Cmd+Shift+F), or
choose Find in Files… from the Edit menu.
In RStudio 1.3, it’s now possible to replace the text you found:

After you’ve done a search, switch to Replace view via the toggle,
enter your new text, and click Replace All. It works with regular
expressions, too.
In order to test it, in RStudio in Windows, when one presses CTRL
+SHIFT
+F
it opens the following

And one can search in the following files

To keep in mind:
You should be aware however that while we do hang out on the community
site occasionally, the best place to make your request heard is on our
Github issue tracker. (Source)
Also, this is how one write a good feature request.