On the Details pane, under the Links Giving Access header, you'll see the People icon, the Links icon, and/or the Email icon. These options vary depending on how you have shared the file or folder.
To stop sharing the file entirely, click Stop sharing.
To delete a sharing link, click the ... next to the link, then click the X next to the link to remove it. (You'll get a prompt asking you if you're sure you want to delete the link - if so, click Delete link.)
To stop sharing with specific people, expand the list under a specific people link and click the X to remove someone.
To change permissions or stop sharing with someone who has direct access, under Direct Access, click the dropdown next to the person's name and choose the option that you want.