How will exercise affect serotonin levels?

Increases it. It has been proven that 8 weeks of exercise is as effective as 8 weeks of antidepressants on reducing symptoms of depression. It is presumed that exercise does this by raising the amount of available serotonin in our brain. Precisely how it does it is still being worked out.
Helps. Usually exercise helps improve and stabilize serotonin levels. This can be very beneficial for sad and depression.