One way:

A1 = N000023004

="N0"&--MID(A1,2,255)

Returns: N023004

On the off-chance that the first letter could possibly be something other
than an "N" and that the second character could possibly be something other
than "0", then this modification to your formula could be used...

=LEFT(A1,1+(MID(A1,2,1)="0"))&--MID(A1,2,255)

Rick