The President can only legalize cannabis through an Executive Order, which is a temporary measure. They can reschedule or deschedule it through the CSA, HHS, and DEA, but Congress would need to pass legislation to decriminalize it and expunge related records.
https://crsreports.congress.gov/product/pdf/LSB/LSB10655
https://www.congress.gov/bill/117th-congress/house-bill/3617