Go to the Accounts Tab of a case that has an Invoice prepared as below:

Click on the Invoice Button as shown below:

Click Next on the First Screen shown below without any changes needed:

Click Next On the second screen shown below without any changes:

On the last screenshot below you can change the Invoice Number sequence marked with the below arrow to for example 5000 and then select Finish. The next number will then be 5001, 5002, etc