AsNoTracking

var entity = context.SystemForms
.AsNoTracking()
.First(f => f.FormName == "Other" && f.ColName == colname);

entity.Content = content;

// 手动告诉 EF：我要更新它
context.SystemForms.Attach(entity);
context.Entry(entity).State = EntityState.Modified;

context.SaveChanges();