How do I change my bill number prefix?

Quick answerIn 2 steps

Bill numbers are in Settings → Documents & series. A prefix that has been used is locked — GST wants numbers consecutive and unique.
A counter's prefix is set when you add it: Settings → Shops & counters → “+ Counter” → “Invoice series prefix”.

WhereSettings → Documents & series in the back office

Learn it properlyCheck document series (website, credit notes, purchase bills)GST rule 46: numbers run in order, once a year, and never go back.Open the lesson →

Also asked as

“invoice series”
“bill numbering format”