You can edit the permissions on a file anytime after you've shared it.

Click on 'Private' next to the file.

The 'Manage Access' box appears to the right:

To stop sharing the file to everyone use the 'Stop sharing' link. That will make the file completely private.

If you want to adjust a specific person's access click on 'Can Edit', the options shown above will then appear.

You can use the 'Stop Sharing' option to prevent that person accessing the document. Alternatively, use 'Change to view only' to prevent them making changes.