Several AI tasks take a clip that is already in your project and hand back a changed version. Framing, timing and motion stay as they were, and only what you asked about is different. The original is never touched: results arrive as new clips in your library.
Select the clip first Click the clip on the timeline, then open Generate. Because you arrived from a selection, the source-video slot already holds that clip. Swap it for another library asset if you meant a different one.
Pick the task Under Video, the action chips name the job. Edit / Restyle changes the shot from a prompt. Reframe changes its aspect ratio, Erase object removes something, Looks applies a fixed transformation, and Dub / Translate puts it in another language.
Say what should change Restyle and Erase take a prompt. For Restyle, describe the result ("make it snow heavily"); for Erase, name the thing to remove and what should be there instead. Reframe, Looks and Dub need no prompt: you pick an aspect ratio, a look or a language.
Check the quoted price Models on these tasks are priced very differently, some flat per run and some per second of your clip, so the number under the button is worth a glance. Draft on a cheap model, then re-run the take you keep on a better one.
Review and place The result lands in your library, and the review picker offers to replace the clip on the timeline, insert it after, or keep it as an asset. Replacing is non-destructive: undo brings the original straight back.
Tips
Only part of the clip needs changing? Split it first: these tasks bill on the length you send, so a shorter selection costs less.
Several models refuse inputs longer than about ten seconds, and a couple need footage at least 720 pixels tall. The refusal happens before anything is charged.
Reframing beats cropping for vertical cutdowns: a crop throws away two thirds of the frame, and usually the subject with it.
A clip with people you want gone is an Erase job rather than a mask job, because the AI eraser handles movement and a painted region cannot.