Press button No1 until the first digit is correct (press 3 times for "3", 4 times for "4" etc)
Press button No2 until the second digit is correct (ditto^^^)
Do the same with buttons 3 and 4 until all 4 code digits are correct, and in the correct order, then press button No5.
If you do it wrong too many times, the radio will lock out permanently, and will have to be replaced, unless you can find someone to repair it