If by 'flight mode' you mean the Beginner, Intermediate, Experienced mode, then you will use the switch that you have assigned to your radio profile channel (8) 'mode'.
If you don't have a switch set for that function you may use the computer keyboard key 'o' which toggles the channel (8) flight mode even though you don't have a controller key assigned to that function. The other channels ( 5,6,7, & 9) also have computer keys. You can see these by opening the simulator 'soft' radio controller gadget. You can use your mouse to select the radio gadget (left side of simulator screen), or you can press the digit '2' on your computer keyboard to open it in the simulator.
PS There are other 'modes' you might be referring to; the controller flight mode (1) or (2) for instance... if so, please clarify which 'mode' you are speaking about and we'll help you. If I don't hear back from you , I'll assume you meant the 'mode' channel (8) 'o' mentioned above.