This is done by going into the "Settings" of your site, the "Tenant" tab, and changing the " Next Workorder Number". (The same may be done for the "Next Quote Number".)
Since Invoice numbers in ServiceBox are an extension of the Work Order they are attached to, the change in your Work Order number will cause the Invoice number generated from the Work Order number to be the same with the addition of the "-1" to it. Therefore, if the next workorder number is changed to 250001 it will cause the first invoice for this workorder to be 250001-1.
The location is highlighted below, near the bottom of the Tenant tab:
