Hi @Kimberley B
If you want to edit permission for files in a team, you could do it on the team's SharePoint Online site.
- Move to SharePoint Online Library
- click ... next to a file to customize permission and click Manage Access
- click ... on the right top of the window and click Advanced settings
- click Stop inheriting Permissions on the ribbon menu
- Click Grant Permissions and add people, select a permission Level and click Share button.
- check groups(members, visitors, owners) or users to remove permission, click Remove User Permission on the ribbon menu and confirm to remove the permission
The file or folders will be access only people have permission
Thank you