Upgrade SQL Server 2016 R Service (in-database)
I faced the very same problem (when trying to follow the steps described in Use sqlBindR.exe to Upgrade an Instance of R Services). I could not find any SQLBindR.exe.

A friend of mine pointed out to me, that I had to explicitly install "Microsoft R Server", not as part of the SQL Server install bits and pieces, but as a separate download.

All options about how you can download Microsoft R Server are described in a blog post at MSDN, Run Microsoft R Server for Windows by Heidi Steen.

Download and install was done in some minutes. SQLBindR took some more minutes.
Voilá: Now the following script is showing me, what I was looking for. :-)

declare @v varchar(100);
exec sp_execute_external_script @language=N'R'
, @script = N'v &lt;- toString(getRversion())'
, @params = N'@v varchar(100) output'
, @v = @v out;

print @v;

-- returns 3.3.2