A new frame (one with more clearance for at least 700 x 30 tires and disc or cantilever brake mounts/bosses), new brakes (either disc or cantilever), and some cross tires. Maybe a new cassette or chainrings, depending on the terrain.
Or if by some miracle you could fit 700 x 30 or larger cross tires without rubbing the chainstays, seatstays, or brake calipers, just buy the tires.