Go to Admin > Config > Clubs (or Workspace > Clubs) > Manage Clubs.
In the Manage Clubs area, you have the option to click onto Edit club, which is in the far right hand side of the club you wish to amend.
In this area, under schedule you are able to remove the incorrect day and click onto the correct day.
Once amended and saved, you will need to re-add the students to the club via Manage Students as they would have been assigned to the original day of the club.