How to uninstall Ruby from /usr/local?

It's not a good idea to uninstall 1.8.6 if it's in /usr/bin. That is owned by the OS and is expected to be there.

If you put /usr/local/bin in your PATH before /usr/bin then things you have installed in /usr/local/bin will be found before any with the same name in /usr/bin, effectively overwriting or updating them, without actually doing so. You can still reach them by explicitly using /usr/bin in your #! interpreter invocation line at the top of your code.

@Anurag recommended using RVM, which I'll second. I use it to manage 1.8.7 and 1.9.1 in addition to the OS's 1.8.6.

Edit: As suggested in comments. This solution is for Linux OS. That too if you have installed ruby manually from package-manager.

If you want to have multiple ruby versions, better to have RVM. In that case you don't need to remove ruby older version.

Still if want to remove then follow the steps below:

First you should find where Ruby is:

whereis ruby

will list all the places where it exists on your system, then you can remove all them explicitly. Or you can use something like this:

rm -rf /usr/local/lib/ruby
rm -rf /usr/lib/ruby
rm -f /usr/local/bin/ruby
rm -f /usr/bin/ruby
rm -f /usr/local/bin/irb
rm -f /usr/bin/irb
rm -f /usr/local/bin/gem
rm -f /usr/bin/gem