Update

var delete = _context.ProductColours.Find(colour.Id);
delete.DeletedAt = DateTime.Now;
_context.Entry(delete).State = EntityState.Modified;
_context.SaveChanges();

Add

Size itemSize = new Size
 {
Description = description,
 IsActive = status == 1,
CreatedAt = DateTime.Now,
 CreatedBy = userId,
ModifiedAt = DateTime.Now
};
_context.Sizes.Add(itemSize);
 _context.SaveChanges();