To change invoice from Credit you have to add Payment Receipt

Sale invoice
– For Sale invoice its called Inward Payment Receipt (When you receive payment from your customer)
Follow instructions on link below to create inward payment receipt
https://gogstbill.com/knowledgebase/how-to-add-inward-payment

Purchase invoice
– For Purchase invoice its called Outward Payment Receipt (When you give payment to your vendor)
Follow instructions on link below to create outward payment receipt
https://gogstbill.com/knowledgebase/how-to-add-outward-payment