Any asset which is created in Marketing Activities should already be in Design Studio.
The form is technically a separate asset from the landing page, so making a change to a form anywhere should affect the landing pages that use it. HOWEVER, editing the form will create drafts on those landing pages. You'll have to find the associated landing pages and approve their drafts for the change to take effect.
It's worth noting that changes made in Design Studio can take a minute or two to translate over to Marketing Activities, so if there don't appear to be drafts created in Marketing Activities, just give it a minute and then refresh the page. If you look at your landing pages in Design Studio, it will show which ones need approval and which are already approved. Look for the yellow "!" off to the right side: