You'll find "relocate colonist" in the colonial section of your doff assignments in nearly every sector and colonial assignment chains in the surrounding nebulae and arms. It reassigns 5 colonists plus you'll need these commodities which are 5 provisions and 1 medical. Doing these assignments nets you 500 dilithium plus the cxp, skills, and BO points.
open the doff window and look at your roster. Along the left hand side, click the tab 'passengers'. It will then display them. From here, you can click on them and dismiss them. Or you can sell them on the exvhange, or you can mail them to yourself to save for later.