cp mario.nes mario.org
hexedit mario.nes

#jump to point 9920
#bricks and tiles are in this area
 9920

#Replace parts and then save and exit

Ctrl+x

#see changes
mednafen mario.nes

#If you need to start over 
cp mario.org mario.nes

#see that area at shell
xxd -ps -s 0x9940 -l 20 mario.nes
#with columns
xxd -ps -s 0x9940 -c 4 -l 20 mario.nes

#in binary
xxd -s 0x9940 -b -l 20 mario.nes