I (sometimes) have a row with numbers from 1 to 5...

What I want is: when I enter value = 3, and that value (3) already exists in
the column, it has to correct that automatically into 4, with or without a
warning.

Sometimes the row don't have values in it at all, if the value 2 is entered
then, it automatically has to change to 1.

In short: it has to change automatically in the first unused value closest
to 0...

Thanks for the help!!!