How to upgrade Joomla from 1.7 to 2.5 using the automatic updater
Joomla version 1.7 has an automatic updater that you can use to upgrade any 1.7 version to 2.5. You don't have to download anything or make any manual changes to the Joomla database or the files.
Log in to the admin panel of your Joomla, then go to the Extension Manager by either clicking on the corresponding button on the admin panel's control panel or by selecting the Extensions menu and then clicking on Extension Manager. On the Extension Manager page click on the Update tab. On the page that opens click on the Purge Cache button (in the upper right corner) and then on the Find Updates button. After you do it the last available Joomla version should be listed on the page. Mark the checkbox in front of it and then click on the Update button in the upper right corner. Joomla will automatically perform the update.
After that click on the Database tab of the Extension Manager and on the page that opens click on the Fix button in the upper right corner. When you do it all the necessary database changes will be performed. In case there were some errors after the completion of the update process, or just to make sure, click on the Discover tab of the Extension Manager, then on the Discover button in the upper right corner to check whether there are any extensions that need to be installed. Once you've done all these things your Joomla should be up to date.
For a more detailed description and for information on some common problems and errors that occur during the update process check out the tutorial on updating Joomla 1.7 to 2.5 with the automatic updater.