On the Website under the 'Settings' Tab, and if you click on 'Invoice Payment Terms and Certificate Numbering'.
You will see options for adding Payment Terms and your bank details to the bottom of all of your invoices.
On the same screen as above you can also adjust what all of your next certificate numbers will be.
For example if your last invoice was '230', you could enter '231' and the software would carry on with your existing invoice numbering. The number you enter here is the next number to be used.
For each new certificate the number is automatically updated.
There is also an option to add a Prefix to the all of the numbers - for example if you entered 'Inv' for the prefix for the Invoice and the Invoice number as '231' the next invoice number would be 'Inv231'.