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:

1. Fixed or date/time based prefix characters
   

   (this is what you want to modify)

2. Fixed or date/time based suffix characters
3. The total number of numerals in the number (padding) ie: 001 or 000001
4. The increment used for the next number in the sequence

5. The next number used for the numeric part of the sequence

6. Whether to allow gaps in the sequence