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How many wires do you currently have and what are you controlling? Stranded wires usually indicate high voltage unless it is small wire like phone cable.
OK the Nest needs solid wires so what you will need to do is pull a set of 18 gauge solid wires between durance ant thermostat.If the existing wire is not stapled to studs in wall, you can connect a new 18/6 control wire to end at furnace and pull through. If existing wire will not pull through you will have to drill new hole in top plate on wall and feed a new wire down wall to thermostat.
If wires will pull through less than an hour.If holes have to be drilled and new wires routed could take 2 to three hours tops.