First, you'll need to configure your SQL server to listen to a specific port by using 'SQL Server Configuration Manager'. Afterwards, you'll need to edit the files below:
Program Files (x86)\Lansweeper\Service\Lansweeperservice.exe.config
Program Files (x86)\Lansweeper\Website\web.configWithin these files, you'll need to search for your SQL instance name and add the custom port number after it using a comma.
Source=localhost\SQLEXPRESS
,portnumber