A scheduled campaign can be changed as long as it hasn't been sent. How you do it depends on what you want to change.
Change the message content
Open the campaign's flow.
Edit the text, image or buttons directly in the nodes.
If you changed a template, submit it to Meta again and wait for the Approved status.
No need to touch the scheduling: the campaign will go out with the updated flow.
Change the audience, date or pacing
Go to Campaigns, open the campaign and click Edit.
Confirm: the campaign goes back to draft while you make changes. It won't be sent until you submit it again.
Change what you need:
the audience (pick another list or segment);
the send date and time;
the pacing, to spread sending over a longer period.
Click Submit to schedule the campaign again.
Don't forget step 4: a campaign left as a draft won't be sent.