How to change which version of Ruby I am using

I need to run 2.5.3. I use brew to manage my ruby installation (as I couldn't get rvm to work on my machine). When I run

$ruby -v

I get

ruby 2.3.7p456 (2018-03-28 revision 63024) [universal.x86_64-darwin18]

However when I attempt to update it using

brew upgrade ruby

I get

Error: ruby 2.6.1 already installed

Why is it so inconsistent on what ruby version I really have installed?