One way to change the financial year would be by changing the prefix that appears in your invoice number.

For instance, if you have been using a prefix for your invoice no&series till now, such as X17-18 and after it your document number, you can change it to X18-19 and then add the document number 1 and a new series starts.

You can add any other prefix and your invoice numbering can start from the beginning.

As an example, check out how ours looks at the moment:

To change your prefix number, make a new invoice or go to an older one and in edit invoice screen, right next to Invoice no you can add your details, as seen below