Pre-requisites: You must have the "Manage purchase documents" user right. 

1. Navigate to Data Management (gear icon) > Document Numbering. 
2. Select the organization you want to edit document numbering for from the Organization panel. 
3. Enter the options you want to use for each document type. You can see an example of your configured number format in the Preview column.
4. When finished, click Save to save the number format. All changes take effect immediately except Reset series at start of year and First number in series, which will take effect at the start of the next year.
5. If you want to copy the number format to another organization, click Copy, then select the organizations and document types to copy number formats to.