Please follow these steps to update:
1. Download the plugin from http://codecanyon.net/downloads
2. Deactivate and uninstall/Delete the version you currently have.
3. Install and activate the version you downloaded from the above link.
You will not loose any forms, submissions, settings or any data when updating!
NOTE: Since version 7.4 you can update the plugin by going to your WordPress installed plugins and then click on update next to NEX-Forms.