If you go into 'build mode' and target a person you should have a "Move" command you can do to them, as passed-out Jun is demonstrating here.
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Hitting that should bring up a list of all settlements you own and then you pick where they will move to. If names of settlements are greyed out, that settlement is at or above the 'population cap' (defaults to your Charisma + 10), you can switch that cap off in the Workshop Framework settings.
Note that if they are currently assigned to any plots when you issue the Move command, it can take a moment to process as it unassigns them from the plots first, so give it some time before you check if it worked