Isn't the thing ?

You can do it in following manner.

When you Press 4th button write some code about blinking first digit.That would be the indication of which digit you are going to change.You right now have given some address to individual digit in your software.Now when you push 1st or 2nd button increace or decrease the value of that address (Here in this case 1st digit's value).Lock the limit between 0 to 9 (as you might already have done).Store that edited value in some specific register that might not affect your present set value.

Now again press the 4th push button & blink second digit and change the value as described above.
Same way you can edit all 4 digits or wahtever the nos. now after edittting all the digits again pressing 4th push button will replace current set value with the edited one.You may store this value in memory IC at this time.

Note:

This is only the hint of how one can edit the digits.But mostly it depends on application demands.

You can reffer your text book or reference book for such examples.

Best wishes

from

Maunik Choksi