The original documentation of Varmilo for its keyboards is rather difficult to understand and plain wrong in some cases. A Reddit thread is incomplete, at least for the 104 keys version. For the 104 keys keyboard, the following combinations can be used:

• Fn + Arrow Right: Switch Backlight mode
• Fn + Arrow Up: increase Backlight
• Fn + Arrow Down: decrease Backlight
• Fn + Right Win: hold for 3 seconds to switch Fn key with Right Win key (to switch back, press Left Win first, then Fn)
• Fn + ESC: hold for 3 seconds to reset settings (if switched with Win key, hold Left Win  + ESC)
• Fn + Left Ctrl: switch Caps Lock and left Ctrl (use Caps Lock key to switch back!)
• Fn + W: hold for 3 seconds to switch to Windows mode
• Fn + A: hold for 3 seconds to switch to Apple mode