The smart guys at NASA designed the trajectory such that as they passed Jupiter, they gained some speed by being dragged along by Jupiter. This is called a gravity assist. Voyager 2 picked up about 18 km/s of velocity from that Jupiter gravity assist. By the time it reached Saturn, Voyager 2 has dropped back down to a little over 16 km/s. It received another gravity assist from Saturn, climbing back up to around 34 km/s. It did this again at Uranus and at Neptune. When it left Neptune, it was traveling at just shy of 29 km/s. Voyager 1 had a different trajectory and did not rendezvous with Uranus or Neptune, it moved outwards at a faster pace.
That was 29 years ago. Every day, since then, the Voyagers have continued traveling in curved paths that takes them ever farther from the Sun and every day, since then, they have lost a little energy and slowed,
There is no "up and down" in space. I suppose you could use "above and below" the ecliptic