On my machine, the upper tension knob is numbered 0 to 10. When using invisible thread on top, it helps to reduce the upper tension because of the stretchiness of the thread. I reduce my upper tension by 2 or 3 numbers.
Many people do not use invisible thread in the bobbon. If you do, be sure to use a metal bobbin, do not overfill the bobbin, and do not wind the bobbin at top speed. Because of the stretchiness of the thread, it can distort a plastic bobbin and get stretched as it is wound. When I use invisible thread in the bobbin, I do not adjust the bobbin tension. As other people have noted, bobbin tension is adjusted by means of a tiny screw on the bobbin; adjusting this screw is fine as long as you know what you are doing, but should not be attempted if you have not read and understood a detailed explanation of safeguards you should take.
I agree that the 3.5 type numbers you are describing probably belong to the stitch length and stitch width knobs. Change one to an extreme and manualy turn the wheel to see what happens. (Manually turning the wheel ensures you won't break a neede if you happen to adjust it to a zigzag while having a straight stitch foot on your machine.)