You can do however you please, the only thing you must make sure is that the new serial number is greater than the old one.
Having said that, I would recommend a timestamp based approach following a scheme like:
xx starts at
00 and is incremented for all edits on that specific day (when editing on another day, you reset
The main advantage of this scheme is, that you also know the date of the last modification of your zone-file at first glance.
It also makes the serial number incrementing more robust.
The alternative is to start with
1 and just increment whenever you edit the file.
If the serial number is already timestamp based (and
2015040500 looks very much like that), you shold probably stick with that decision (even if not made by you), and use the logical successor