While President, the President has authority to classify and declassify. (There is a specific exception re declassification of nuclear secrets.)
Upon the moment after being President, the following may apply . . .
https://www.ecfr.gov/current/title-32/subtitle-B/chapter-XX/part-2001/subpart-D
Subpart D—Declassification
§ 2001.30 Automatic declassification.
(b) Presidential papers, materials, and records. The Archivist shall establish procedures for the declassification of presidential, vice-presidential, or White House materials transferred to the legal custody of NARA or maintained in the presidential libraries.
[def: NARA - National Archives and Records Administration]