This means that he will not change what he has said. Alternate translation: "will not change his mind" or "will not change what he has said" (See: [[rc://en/ta/man/translate/figs-explicit]])
The idea of being priest is understood from the previous phrase. It can be stated clearly here. Alternate translation: "after the manner that Melchizedek was priest" or "in the same way that Melchizedek was priest" (See: [[rc://en/ta/man/translate/figs-ellipsis]])