Edit server.properties
From your server's CrashPanel (e.g., from the console), on the left, in the Server section, you will see the Properties page.
Here, you can easily modify all the options you would find in the normal server.properties file in a grouped manner.
It can generally be dangerous to modify these options without knowing what you are doing.
Note that to save a specific category (such as Player properties), you must click the SAVE CHANGES button.