The way CBD interacts with your serotonergic system is very complicated. In short, there are many ways CBD may increase serotonin by activating the serotonin receptors but not causing re-uptake, thus increasing the amount of serotonin in the synapses.
CBD is a very interesting substance. My main gripe is that, to get antidepressant and anxiolytic effects, dosages are higher than what can conveniently (and affordably) be achieved with the strength of currently available drops/flowers on the market.