You’ll want to follow the instructions here.
In the Config.h file, before you load to your arduino, you’ll change:
// #define ENABLE_DUAL_AXIS
to
#define ENABLE_DUAL_AXIS (just remove the slashes to uncomment)
also
#define DUAL_AXIS_SELECT X_AXIS
to
#define DUAL_AXIS_SELECT Y_AXIS
You’ll comment line 662 with two slashes so it will look like:
//#define DUAL_AXIS_CONFIG_PROTONEER_V3_51
And you’ll uncomment line 675, so it will look like this:
#define DUAL_AXIS_CONFIG_CNC_SHIELD_CLONE
Save and then upload as explained in the link above.
Then, without power…
You’ll want to switch the X & Y stepper connections and limit switches at the board.
Power up and test the X & Y movements before you try homing or anything.
Good luck!