Short answer: Usually no. If you pay premiums pre-tax through a Section 125 plan, your election is locked for the plan year unless you have a qualifying life event. Without a QLE, you generally must wait until open enrollment.
Because most employees pay their premium share pre-tax under a Section 125 (cafeteria) plan, IRS rules make those elections irrevocable for the plan year. You can only add, drop, or change coverage mid-year if you have a permitted change in status, a qualifying life event like marriage, a new child, or loss/gain of other coverage, and the change must be consistent with the event. Absent a QLE, changes wait until the next open enrollment. (Plans not run through Section 125 have more flexibility, but most employer plans use it.)