Upgrading VSNS Lemon should be relatively simple and painless. 1) Upgrading consists of two steps: uploading the replacement files and running the upgrade script.
The first step is easy. 2) Download the latest release of VSNS Lemon. Upload all of the new files to your blog directory, overwriting every file except settings.php.
All upgrade scripts are located in the aptly-named upgrade directory and are named vsns_X.x.x_to_vsns_Y.y.y.php, where X.x.x and Y.y.y are version numbers. If you are performing multiple upgrades at once, you'll need to run each upgrade file in order. So if you want to upgrade from version 3.2.1 to 4.0, you must first run vsns_3.2.1_to_3.2.2.php and then run vsns_3.2.2_to_4.0.php.
The upgrade script will make any necessary changes to the database and update your version number. If required, it also writes a new settings.php file while preserving your information. For this reason, make sure that settings.php is writable by PHP when you perform an upgrade.