Do this via the menu sequence
Settings -> Technical -> Sequences and Identifiers -> Sequences
The example below is for Invoices via the Sales Journal, but if you
open the sequence for Purchase Orders you will see the same type of
record:
You can see the things you can modify about a sequence are:
- Fixed or date/time based prefix characters
(this is what you want to modify)
- Fixed or date/time based suffix characters
- The total number of numerals in the number (padding) ie: 001 or 000001
- The increment used for the next number in the sequence
The next number used for the numeric part of the sequence
Whether to allow gaps in the sequence