cannot "Add or Update" Entity because Primery key cannot be changed

I am implementing an import routine, where a user pastes a specific formatted string into an input field, which in turn gets tranformated into an entity and then put into a database.

The algorithm checks if the entity already exists and either tries to update it or insert it into the database. Inserting works fine - updating fails.

//considered existing if Name and owning user match.
if (db.Captains.Any(cpt => cpt.Name == captain.Name && cpt.User.Id == UserId))
{
    var captainToUpdate = db.Captains.Where(cpt => cpt.Name == captain.Name && cpt.User.Id == UserId).SingleOrDefault();

    db.Entry(captainToUpdate).CurrentValues.SetValues(captain);
    db.Entry(captainToUpdate).State = EntityState.Modified;
    await db.SaveChangesAsync();
 }

The problem at hand is, that written like this, it tries to update the primary key as well, (captain Id is 0, whereas captainToUpdate Id is already set) which results in an exception The property 'Id' is part of the object's key information and cannot be modified.

What do I need to change, so the enttiy gets updated properly. If it can be avoided I don't want to update every property by hand, because the table Captain contains 30ish columns.