Presidents can declassify material but it doesn’t seem as though the material has been declassified – as the Washington Post makes clear by carefully not citing the actual information and federal agencies still treating it as such. If it was declassified a FOIA request could be used to get it.
It seems Trump never intentionally declassified this material and that it still is classified. He had the legal authority to do it but he didn’t – he gave classified information to Russia. The statutes he violated are easily accessible but the point is clear without the citation.