need of code for 10 digit text editing using 89c51

Code C - [expand]
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....
....
switch (keypress)
{ 
  case  0:  //up 
  {
      if (last_value==0x39)//if last value is '9' then set as 'A'
      {
          last_value=0x41;
      }
      else
      if (last_value==0x5A)//if last value is 'Z' then set as '0' (zero)
      {
          last_value=0x30;
      }
      else last_value++;   //else, do normal increment
      break;
  };
  case  1:  //down
  {
      if (last_value==0x41)//if last value is 'A' then set as '9'
      {
          last_value=0x39;
      }
      else
      if (last_value==0x30)  //if last value is '0' (zero) then set as 'Z'
      {
          last_value=0x5A;
      }
      else last_value--; //else, do normal decrement
      break;
  };
.... 
....
.... 
};