Yes, via the Arduino IDE. Select Ardunio Uno / atmega 328 as the board type and CPU.
And yes, you can go back to the official firmware if you goof something with your custom build.
See this thread for more information on both flashing and rolling back.
The main gotcha is noted in @WillAdams’s first comment:
Note that depending on board version you may need to hold down the Z-axis homing switch (newer revisions) or a “Program” button on the board itself (older boards), and keep it pressed until the process completes, just like carbideupdater
Also note that (I believe…) you will need to update the EEPROM values for grbl after you re-flash. Carbide motion can push them all automatically, or you can do it manually from any gcode sender that has a terminal.
These are the values I have documented. The only change I made to the below was to update the Min and Max spindle speed according to my VFD.
Note: sending $$ to GRBL will list out all current settings
$132=80 //[X,Y,Z] Max travel, mm
$131=465 //[X,Y,Z] Max travel, mm
$130=425 //[X,Y,Z] Max travel, mm
$122=400 //[X,Y,Z] Acceleration, mm/sec^2
$121=400 //[X,Y,Z] Acceleration, mm/sec^2
$120=400 //[X,Y,Z] Acceleration, mm/sec^2
$112=5000 //[X,Y,Z] Max rate, mm/min
$111=5000 //[X,Y,Z] Max rate, mm/min
$110=5000 //[X,Y,Z] Max rate, mm/min
$102=40 //[X,Y,Z] steps/mm
$101=40 //[X,Y,Z] steps/mm
$100=40 //[X,Y,Z] steps/mm
$32=0 //Laser mode, boolean
$31=0 //Min spindle speed, RPM
$30=1000 //Max spindle speed, RPM
$27=5 //Homing pull-off, mm
$26=25 //Homing debounce, milliseconds
$25=1000 //Homing seek, mm/min
$24=100 //Homing feed, mm/min
$23=0 //Homing dir invert, mask
$22=1 //Homing cycle, boolean
$21=0 //Hard limits, boolean
$20=0 //Soft limits, boolean
$13=0 //Report inches, boolean
$12=.01 //Arc tolerance, mm
$11=.02 //Junction deviation, mm
$10=255 //Status report, mask
$6=0 //Probe pin invert, boolean
$5=0 //Limit pins invert, boolean
$4=0 //Step enable invert, boolean
$3=6 //Direction port invert, mask
$2=0 //Step port invert, mask
$1=255 //Step idle delay, millisecond
$0=10 //Step pulse, microseconds