Slowly, and never with the opener. Disconnect the opener first with the release cord, then work the bottom edge with warm water or a heat source kept moving, freeing the seal a section at a time, because prying against a frozen seal tears the seal and can bend the bottom section. Once free, dry the edge and check the seal's condition: a seal that froze hard has usually been flattened or cracked for a while, and replacement before the next deep freeze is the actual fix. If the door still will not lift freely by hand after the seal releases, the problem has moved to springs or tracks, and that is the call, not more force.
Prefer to talk later? Leave your details and an independent local pro will call you back. No charge to you, no obligation.
Piscataquog Garage Door Co is a free referral service. Submitting this form connects you with independent local contractors who provide the work.