Settings > Billing > Order Settings

You can assign the setting for the document format of your order numbers here. 
The variable {NUMBER} is important here. This variable automatically draws the next number. 

You have already made these settings at the beginning with the Getting Started Wizard. However, if you need to make a change, you can do so under the following path: 

Settings > Billing > Order Settings

In general, you can assign your document format however you like. It is also not necessary to enter a year. 

However, the variable {NUMBER} is important. This variable automatically draws the next number. 

Order Format:  Enter your desired order number here.

Next Order Number: Here you can enter which consecutive number should be assigned next. This number is appended to the previously assigned order number.