How do I change the text of a script with a plugin?

Assuming it's the Source of the script you wish to edit, not the actual font of the script-editor.

You can edit the Source of a script by accessing it through the Source-property (script.Source). It's important to note that regular scripts do not have the security clearance to set .Source anymore, but plugins (and commandline(?)) do.

I assume you want to replace the default "Hello world!"-print, so I made an example of how you could achieve that.

mySource = "-- This script was edited with ChipioIndustries' plugin. "
replaceTemplate = [[print 'Hello world!'\n]] -- This is the default template.

-- Adding a ChildAdded-event to ServerScriptService, so this can be applied to all newly created children (or newly re-parented)
game:GetService("ServerScriptService").ChildAdded:connect(function(child)
    if child:IsA("BaseScript") or child:IsA("ModuleScript") then -- Makes sure it's a script
        if child.Source == replaceTemplate then -- Checks if source matches template
            child.Source = mySource -- replaces source.
        end
    end
end)

HOWEVER, if you did mean to edit the font, you can take use of settings(). I'll post back with results if I find anything further.