How to change the default menu and signature by Fchat? 

The menu is displayed in the chatbox of customers when they message your fanpage. 

Each fanpage can create up to 3 main menus. Fanpages using the free version of Fchat will have the 3rd menu as the default signature. 

To add / edit menu, go to: Page Settings >> Menu  

You can completely edit the default signature "Chatbot by Fchat.co" on the main menu with the paid version. 

See Fchat's service packages at: https://fchat.co/price