It's generally recommended to have a portion of the bio-balls submerged. What I see in your photo looks like a pretty good level. You cold even add a bit more water, that is as long as you don't exceed the safe amount where the sump would overflow when you turn the pump off. Bio-balls work best when they have some exposure to air, but they are still effective when completely submerged in water. When I do a W/C, my bio-balls are almost 100% covered with water before I turn the pump on. Once the pump is on, they are about 25% covered.
To the OP,
As others have stated, the process used to change your water will vary depending on you water volume in the main tank vs. that of the sump. I do W/C's 2x a week because I can only easily change about 15-20% of the complete system by draining the sump and refilling it. To be specific, I have a 150 and a 90 running off of the same sump. The sump is a 125 standard tank, but since the filtration system sits fully beneath the top of the tank/sump, I can only fit about 80 gallons in there (while the pump is running - probably 110-120 gallons when its turned off). Also, I use the pump to push the water out of the sump, and since the intake of the pump is about 1-1/2" from the bottom, I can't drain ALL of the water.
So, my W/C's are very easy, since all I have to do is connect a hose and turn a valve to drain the sump - but they must be done twice as often as I used to have to do when I used a Python to siphon water out of the main tanks. (pre-sump)
Another thing, if you have your sump returns and powerheads (UG jets in some people's cases) positioned well, you can get away with almost never having to vacuum your substrate (unless you have gravel). My tank maintenance consists of mothing more than W/C's as described above, filter cleaning once a month, and a little algae scraping. HTH, good luck.