Inspect the sequence
Use Timeline → Sequences → Sequence Settings or find Sequence Settings in command search. Read the current dimensions and frame rate before deciding what to change. The canvas describes the sequence image; the source files can have different sizes and rates.
Choose for the intended film
For a landscape project, use a landscape canvas. For a vertical deliverable, create a format copy and review framing in that copy. Changing a canvas affects composition: a centered subject, title, or picture-in-picture layout may need a new placement.
Keep the sequence rate deliberate. A source rate and a delivery rate are not interchangeable labels. Slow motion and retiming also change which source pictures appear at sequence times.
Check after a change
Review the first and last frame of the sequence, representative edits, title safe areas, and any animation. Then inspect Deliver: saved output dimensions can be fixed rather than following the current canvas.
Do not assume that changing a preset repairs an already awkward crop. Use per-shot positioning and inspect the actual exported file.