Add Roles to User using checkboxes

I am developing a Razor application and I need the Admin to assign or update roles of the Users. I am using a checkbox to carry out this action. So far, I have been able to populate the view to show the users and their respective roles ticked in the checkbox, but I haven’t been able to update their roles, as anytime a checkbox is ticked to add a role, it doesn’t add the role to the user. I think this line is the culprit: var selectedRoles = model.Where(x => x.Selected).Select(y => y.RoleName);.