This is just an example of how to resolve this issue. The paths and file names might be different on your system, but you should get the idea from here:
# First locate the original ruby
> which ruby
/usr/bin/ruby # <- Your path might be different
# Then locate ruby19
> which ruby19
/usr/bin/ruby19
# Move the old ruby out of the way
> mv /usr/bin/ruby /usr/bin/ruby_old
# Link ruby to the new ruby (ruby19)
# ln -s is used to create a new symbolic link. See "man ln" for more info.
> cd /usr/bin
> ln -s ruby19 ruby
Now you should have:
/usr/bin/ruby_old # The old executable
/usr/bin/ruby -> /usr/bin/ruby19 # The new link
/usr/bin/ruby19 # The new executable
Note: it's easy to break your system ruby if you're not careful using this method. That's why RVM is usually a better solution if you have the choice. You can leave a comment if something breaks, and I will try and improve the instructions.