The linoleum around my toilet is turning dark, I am afraid the wax seal is leaking. Can you explain how to change a wax seal?

Yes you are likely right about the wax seal. I have also seen sweating toiler tanks and leaking tank bolts cause wet floors so have a close look there before removing the toilet. To change the seal start by turning off and disconnect the water supply, hopefully there is a shut off valve. Then flush the toilet and hold the flush handle down until no more water is moving. Next get a plastic disposable pail to bail out the remaining water in the bowl. You can also use a plunger to try to force the water out of the toilet. Now loosen the floor bolts and remove the nuts. Remove the tank lid so it does not fall and break. Get a piece of cardboard to lay the toilet on. Using a friends help gently lift the toilet straight up and place on the cardboard. Finally clean the wax seal in the toilet and flange. Finally put in the new seal and once again gently replace the toilet straight back into position careful not to move the toilet too much in the process. Reassemble.