It is not a simple task I am afraid. There are a number of tools available on the JED but all have their drawbacks. Not all menu;s will work as expected, not all modules will have their data migrate.
Here is a little tutorial – http://www.jomsocial.com/blog/migrating-joomla-15-to-joomla-25-a-guided-tutorial.html
Here are the tools to do the job – http://extensions.joomla.org/extensions/migration-a-conversion/joomla-migration
But you will run into problems. I suggest you work from an copy of your live site on your local machine. I use Akeeba backup to take a copy then install it under WAMP on my local machine where I do all the work. It can take a few hours up to a few days to migrate a site well. You also need to check that all of the components you need are available or have suitable alternatives for J2.5.