Take the program from the arduino board
You can retrieve the program currently in your Arduino with the
avrdude tool. This tool comes with the Arduino installation. The way
to call it is somewhat OS-dependent. On Ubuntu, for an Arduino Uno, I
type:

avrdude -p atmega328p -c arduino -P /dev/ttyACM0 -b 115200 -D -U flash:r:prog.hex:i


This creates the file prog.hex with your program in Intel
HEX format. You can save this
file if you want to re-install it on the Arduino later. You can also
turn this file into an assembly listing with avr-objdump:

avr-objdump -m avr -Dz prog.hex &gt; prog.lst


You may be able to make some sense of this listing if


you are well versed in AVR assembly language
you have lots of spare time


As others have said, you cannot expect to get the original C++ source
back.