To change mixer type, connect to board with some serial terminal at 115200,8n1 (I use herculessetup on windows) and type the 2-letter combo for the mix you want. The board will reboot with new mixer, confirm it in GUI.
MULTITYPE_TRI = 1, // XA
MULTITYPE_QUADP = 2, // XB
MULTITYPE_QUADX = 3, // XC
MULTITYPE_BI = 4, // XD
MULTITYPE_GIMBAL = 5, // XE
MULTITYPE_Y6 = 6, // XF
MULTITYPE_HEX6 = 7, // XG
MULTITYPE_FLYING_WING = 8, // XH
MULTITYPE_Y4 = 9, // XI
MULTITYPE_HEX6X = 10, // XJ
MULTITYPE_OCTOX8 = 11, // XK
MULTITYPE_OCTOFLATP = 12, // XL the GUI is the same for all 8 motor configs
MULTITYPE_OCTOFLATX = 13, // XM the GUI is the same for all 8 motor configs
// XN missing for some reason??
MULTITYPE_VTAIL4 = 15, // XO
Oh, and in TRI mode, the motor order changes, too.
Motor #1 becomes the tilt servo, then motors 3.4.5 are for the tri outputs.
Need to update manual for this!