Changes come from design revisions, owner requests and hidden conditions. The contract says how to price a change and who must sign it before the work starts.
Example: the owner adds two volleyball sleeves after the floor is poured. The contractor prices the coring, the sleeves and the labor, and submits a change order request.
Price a change like a small bid. Take off the added and deleted work, and show labor, material and markup. Check the contract for allowed markup rates and notice deadlines. Do not start changed work on a verbal instruction if the contract requires a written one.