Several notes:
Make sure you first get a list of all installed versions. I actually had the version I wanted to downgrade to already installed, but ended up uninstalling that as well. To see the list of all your versions do:
sudo gem list cocoapods
Then when you want to delete a version, specify that version.
sudo gem uninstall cocoapods -v 1.6.2
You could remove the version specifier -v 1.6.2 and that would delete all versions:
You may try all this and still see that the Cocoapods you expected is still installed. If that's the case then it might be because Cocoaposa is stored in a different directory.
sudo gem uninstall -n /usr/local/bin cocoapods -v 1.6.2
Then you will have to also install it in a different directory, otherwise you may get an error saying You don't have write permissions for the /usr/bin directory
sudo gem install -n /usr/local/bin cocoapods -v 1.6.1
To check which version is your default do:
pod --version
For more on the directory problem see here