MAC - Unable to upgrade to Ruby latest version

I'm trying to upgrade to latest version of ruby but could not upgrade it.
Here is the latest command -

Ms-MacBook-Pro:tests p$ ruby --version
ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin16]
Ms-MacBook-Pro:tests p$ brew upgrade ruby
Error: ruby 2.5.3_1 already installed

The version command says ruby 2.0.0p648 but when I enter 'brew upgrade ruby' it says - Error: ruby 2.5.3_1 already installed

MAC Version - MAC OS Sierra