Installer installed; but ruby -v still gives me old version

[curiousx...@gmail.com]

ruby version: 2.1.5
gem version: 2.6.12
My OS: Windows 8.1

Yo,

I am trying to update Ruby to a version which can use the nokogiri gem (2.2 or higher).

I've used the installer to download C:\Ruby24-x64. All the files are present and correct. I've also checked the PATH is set up correctly.

But when I go back to my command prompt - I've closed and opened it again - it still gives me version 2.1.5.

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What have I done wrong? I feel like an idiot - I think I've followed everything correctly.