Step 1: Pick the Joomla 2.5 component which you want to install. You can search for one in the Joomla extensions directory:
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http://extensions.joomla.org/
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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 component archive file and click on Upload & Install.
Step 6: Once the component is installed you can find it under the Components area.