Log in to the Admin and go to the category "Store" > to the category subitem "Stores".
Press the green pencil icon button "Edit".
Then go to the "Settings" tab > to the "Cash journal and cash balances" section > to the "Prefix for invoice number" or "Suffix for invoice number" setting.
Prefix for invoice number setting:
Here you can define - also with the help of placeholders - the beginning of your invoice number.
Allowed placeholders are:
%y% = two-digit year,
%Y% = year with four digits,
%m% = two-digit month,
%z% = day of the year.
Suffix setting for invoice number:
Here you can define the end of your invoice number.