Updating the script can be a time consuming process depending on how many files you have modified and what has changed in the update. Unless there is a critical security issue update, or there is a feature you want, it would save time to stay on the version you are on currently.
Download and extract the new version of Quform from your CodeCanyon Downloads page.
Make the same changes to the files in the new package that you did to the previous Quform version. Mainly this will be copying over the settings from process.php into the new process.php file. Copy email recipients, element configuration and anything else you changed. The form HTML on your site can probably stay the same. If you have made any CSS changes in base.css make the same changes to the new base.css file.
On your server rename the quform folder to quform2, which means you have a backup of it for now. Then upload the quform folder from the new package in its place. Test the form again and sort any issues if needed.