Step 1:
Log into your game panel and click 'stop' on your service.
Step 2:
Select File Manager from the icons
Step 3:
Once in File Manager navigate to \settings\Saves\
Step 4:
You will see maps such as the following:
Step 5:
Find the Map that you wish to load and note down its full name
Step 6:
From the list of icons choose 'Configuration Files' then use the configuration editor to edit SpaceEngineers-Dedicated.cfg
Step 7:
You will see something along these lines
Step 8:
Change the 'Load World' option to be the world you noted down
Step 9:
Save the configuration file and start your server