If you have slotted cable stops, you can slip the housing out of there, and lube/deburr the inner cable. To create slack you need to either disengage the QR for brakes or shift to biggest cog/chainring and WITHOUT pedaling release the cable with the shifter. The housing should have enough slack to slip out of the cable stop on the frame. If that doesn't do it, take the wheels off, and squeeze the brakes together with your hand or hold the derailleur up at the biggest cog/chainring with your free hand.

If you have low normal deraileurs, trade largest for smallest on the gears.