The shortest path, not requiring repeat runs, is probably the (in)famous sub-replace expression.
A recent q&a asked a similar question regarding doing a replacement on just a portion of a line. In fact, they also wanted to operate on text inside parentheses. (This could arguably be considered a duplicate question...but I won't argue that here.) I posted a pretty extensive answer on using sub-replace expressions which is a method for running an arbitrary Vim expression as the replacement part of a substitution.
Some people find this approach complicated but I don't think it's too bad once you break it down....which I tried my best to do.
Briefly, either of these will work for your question...
:s#(\([^)]\+\))#\='(' . substitute(submatch(1), '/', '.', 'g') . ')'#
:s#(\zs\([^)]\+\)\ze)#\=substitute(submatch(1), '/', '.', 'g')#
And there are other variations, too. These are a little stretched out for readability (maybe). It also depends on how conservative/error-proof you want the solution to be.
Rather than repeat here what I said in that answer I suggest you read the original: How to run a substitute command on only a certain part of the line