In the manual it tells how to erase an incorrect code. BTW, an incorrect code will stay on the display until it is erased.
To erase(if it will at least...) with the radio on and the code displayed, press and hold the band button until it disappears, at which point you can enter the correct code.
As far as what Johnny was saying about the fuses.... The radio uses 2 power inputs. Normal operating power(switched with the ignition switch) is on fuse #8- remove it 1st, keep it out. The radio memory is powered from fuse #5 - remove it 2nd. Install in reverse order.
Electrically speaking, if you have the key switch in the OFF or PARK,(not R or ON) there is no power to fuse #8 anyway, so go ahead and remove #5 to remove power.