by the emulator menu I could not. so I had to edit the file

for anyone looking to remap buttons on a SNES controller:

1º create file in:
\192.168.1.xxx\share\system.config\retroarch\config\remaps\FCEUmm
name file: FCEUmm.rmp

input_player1_btn_b = "8"
input_player1_btn_y = "0"
input_player1_btn_a = "10"
input_player1_btn_x = "9"
input_player2_btn_b = "8"
input_player2_btn_y = "0"
input_player2_btn_a = "10"
input_player2_btn_x = "9"
input_libretro_device_p1 = "1"
input_player1_analog_dpad_mode = "0"
input_libretro_device_p2 = "1"
input_player2_analog_dpad_mode = "0"
input_libretro_device_p3 = "1"
input_player3_analog_dpad_mode = "0"
input_libretro_device_p4 = "1"
input_player4_analog_dpad_mode = "0"
input_libretro_device_p5 = "1"
input_player5_analog_dpad_mode = "0"

in SNES controller:

0= botton1
8= botton2
3= start
6=left
7=right
5=up
4=down

regards