Sometimes, after updating your Cocoapods to the lastest version (find out how to do it here), you don’t like it and want to downgrade it back. Or you just want to install a specific version (not the latest).
How to do it?

Check your current pod version:

$ pod --version

Check your installed pod versions:

$ gem list cocoapods

You may see the result like this

*** LOCAL GEMS ***

cocoapods (1.6.0, 1.5.3)
cocoapods-core (1.6.0, 1.5.3, 0.25.0)
cocoapods-deintegrate (1.0.2)
cocoapods-downloader (1.2.2, 0.2.0)
cocoapods-plugins (1.0.0)
cocoapods-search (1.0.0)
cocoapods-stats (1.1.0)
cocoapods-trunk (1.3.1)
cocoapods-try (1.1.0)

It means you have 2 versions installed in your machine: 1.6.0 and 1.5.3.

If you want to keep 1 version, for example: 1.5.3, try to uninstall version 1.6.0 by:

$ gem uninstall cocoapods -v 1.6.0

You may need sudo in your command.

Or just remove all your installed versions and install a new one:

$ gem uninstall cocoapods 
$ gem install cocoapods -v 1.5.2

Have fun!