Editing server.properties file

I'm having problems with the server where the server is running in offline/insecure mode, and the log said to disable this I will need to edit to server.properties file. How do you edit the file in aternos?

Quote from Roman

offline-mode = cracked mode

You can change that setting on the options page. Only enable cracked mode if people with a cracked minecraft account are allowed to play on your server.

Also please note that changing that setting may cause loosing your playerdata like inventories, your position etc.

Thanks