In Bidrento you have 2 options for generating rental invoices in the beginning of the month - either you have enabled automatical invoice generation for the beginning of each month, or you can generate invoices manually, when you go to invoices section.

In case you already have generated invoices, but you accidentaly delete one or more of them, you can click on the blue "Generate invoices" again. Bidrento won't generate identical invoices for the second time, and will only generate the missing ones.