Nested if and Left and right statements

On your formula, you just need to remove the A1 in the middle, eg.

=IF(LEFT(A1)="1",LEFT(A1 +1),IF(LEFT(A1)="2",LEFT(A1+1),A1))

or shorten to..

=IF(OR(LEFT(A1)={"1","2"}),LEFT(A1+1),A1)