At the same time, new air or water fills the space that was vacated when the heated molecules rose up. The air or water that gets pushed out of the way falls down. This sets up a circular motion. Air or water is heated at the bottom, travels to the top, cools, gets denser, falls, is heated again and the whole cycle starts again. Convection does not occur in space because there is no gravity.
Ovens work by convection.
The heating coils at the bottom of the oven heat the air which climbs to the top, cools slightly, and falls down again.