The program will automatically follow the numbering once you've created an invoice. If you're referring to change the starting invoice number, you can fix it by creating a new one and set the number to the desired next number. Once you do it, the sequence will follow the last invoice number created.
We'll need to turn on first the Custom transaction numbers feature to make it possible. It also helps to change the number of previous invoices. Here's how:
Go to the Gear icon.
Select Account and Settings under Your Company.
In the Sales tab, enable the Custom transaction numbers feature under the Sales form content section.
Click Save. Then, Done.
Once done, we'll see an Invoice no. field where we can enter our own numbering preferences. We can also open the past invoices and modify the number. Let's take a look at this sample screenshot:
That will do. Also, I'd like to include some articles that will guide you through customizing, attaching files, and emailing sales forms in QBO: