He can change the schedule through the HHS and DEA, without congressional approval, and he has already instructed them to do so. The HHS recommended schedule 3, but the DEA continues to propose schedule 1. Harris has been pressing the DEA, as well as identifying opportunities to pardon more people with marijuana convictions.
So, what the hell is the DEA waiting on? Biden appointed the head of the DEA. Changing the schedule for marijuana should have been a top priority for any DEA selection.
Descheduling would still require approvals from both organizations, unless done by executive order. Executive orders are temporary solutions to allow proper legislative action. If he descheduled with executive order, and the HHS and DEA didn’t not agree, the schedule would return at the end of the order. That would be highly disruptive to the industry.