If you want to install some previous version of RubyGem or for what ever reason, want to install RubyGem from source, perform following steps

1. Login to your server using putty
2. Go to RubyForge and copy the URL of RubyGem of your choice (e.g RubyGem-1.4.2)
3. Download it to your server using the following command
   wget http://rubyforge.org/frs/download.php/73882/rubygems-1.4.2.tgz
4. Uncompress your RubyGem archive using the following command
   tar -xvf rubygems-1.4.2.tgz
5. cd rubygems-1.4.2
6. ruby setup.rb –no-format-executable

Note that you need to run these commands as super/root user otherwise you'll get an error on step 6.