Here's how it works. Heat is transferred from the Earth's core which heats the magma above it, and this (comparatively) hot magma rises to the top, much like how hot air rises, and then since it is further away from Earth's core than it was when it was first heated, it cools, and falls back down to the bottom, much like how cold air moves down in the atmosphere, and now more hot magma moves to the top after being heated, cools, and the cycle continues.
Let me know if you need further explanation.
