The upgrade leaves your existing data intact. There are a few exceptions, which are outlined here: http://lansweeper.com/forum/yaf_postst6619_Lansweeper-5-0-beta.aspx
Upgrade instructions can be found on page 165 and beyond of our online documentation: http://www.lansweeper.com/documentation.pdf
We recommend performing a database backup before upgrading, just to be safe.
To take a backup of your database right-click on the lansweeperdb database in SQL Server Management Studio and select Tasks\Back Up. If SQL Server Management Studio isn’t installed on your Lansweeper server, we recommend downloading it online. (You will need to perform a search for the Management Studio that’s compatible with your particular edition and version of SQL Server.)