Joomla 1.7.0 was released today. Joomla 1.7.0 is a direct upgrade of 1.6.5. All users of 1.6 are recommend to upgrade, since 1.7.0 fixes several security vulnerabilities in all previous versions of 1.6.

Note, make sure that you're running 1.6.5 before upgrading. Here's the tutorial on how to upgrade to 1.6.5:
https://www.ostraining.com/blog/joomla/how-to-upgrade-update-your-joomla-version-using-admin-tools-extension/

Once you're at 1.6.5 you can use this tutorial to upgrade to 1.7.0:
https://www.ostraining.com/blog/joomla/update-joomla-16-17/

Please make sure you make a backup before beginning:
https://www.ostraining.com/blog/joomla/how-to-backup-a-joomla-16-site/

For those just starting in Joomla, here is a tutorial on how to install Joomla 1.7:
https://www.ostraining.com/blog/joomla/tutorial-how-to-install-joomla-17/

For 1.5 users we generally recommend that they wait until Joomla 1.8 since Joomla 1.5 will be supported for another 8 months. However, if there is a compelling reason to migrate to 1.7 now, here's is the tutorial:
https://www.ostraining.com/blog/joomla/migrating-from-joomla-15-to-joomla-17/