Step 1: You can find the Joomla 2.5 plugins which you want to install. You can search for one in the Joomla plugin directory:
Note : Make sure that it is compatible with Joomla 2.5.
Step 2: Download it on your local computer
Step 3: Log in your Joomla 2.5 admin area.
Step 4: Navigate to Extensions -> Extension Manager.
Step 5: Click on the Browse button, find the plugin archive file and click on Upload & Install.
Step 6: Once the plugin is installed you can manage it from the Extensions -> Plugin Manager area.