Short answer: drop the PDF, choose 90°, 180° or 270° and press “Rotate and download”. Every page turns by default; switch to “Only these pages” to fix a single sideways scan. The rotation is written into the page tree, so it survives printing and email.
Why a rotated page comes back sideways somewhere else
Some tools rotate the page view and some rotate the page content. Only the second kind travels: a page rotated here carries the angle in the file, so every viewer, printer and portal upload shows it the same way. A viewer-side rotate is lost as soon as the file is opened elsewhere, which is why a scan can look fixed on your phone and crooked on a government portal.
Mixed batches
Scans often arrive with a few pages upright and a few upside down. Rotate 180° on the odd pages first, then check the whole file in the preview on the organize page — that tool shows every page in output order, so a wrong turn is visible before you send the document anywhere.