unable to install or reinstall/remove Ruby,Rails on OS X EI Capitan

$ gem update

Updating installed gems ERROR: While executing gem ... (Gem::RemoteFetcher::FetchError) bad response Not Found 404 (http://ift.tt/Td9dpv)

$ gem install rails

ERROR: Error installing rails: activesupport requires Ruby version >= 2.2.2.

$ gem install bundler

ERROR: While executing gem ... (Errno::EACCES) Permission denied - /Library/Ruby/Gems/2.0.0/gems/bundler-1.12.5/.codeclimate.yml

$ gem install rails -v 4.2.6

Fetching: activesupport-4.2.6.gem (100%) Successfully installed activesupport-4.2.6 ERROR: While executing gem ... (Gem::FilePermissionError) You don't have write permissions for the /usr/bin directory.

Please help me , Thanks