If you're already using AdWords Editor, you'll be prompted to upgrade automatically. This update won't require you to re-download your account, and it won't affect your comments and unposted changes. When prompted simply choose the 'Update without Backup' option. Then, after you upgrade, your account will already be in the new version of AdWords Editor.
If you're not already using AdWords Editor, you can visit our website and click 'Download AdWords Editor.' To learn more about the benefits of using AdWords Editor to manage your ad campaigns offline, please refer to these frequently asked questions. Also, comprehensive instructions and tips are available in our Help Center.