Update the constitutional committee is a governance action that changes the composition of the Constitutional Committee: it can add members, remove members, or change the threshold of committee votes required for the committee to approve an action.
It is voted on by SPOs and DReps. The committee itself does not vote, because it is the subject of the action. Together with the motion of no confidence, this action is how the community keeps the committee accountable: membership is granted and revoked on-chain, not appointed permanently.