ZA is suggesting what I would be doing. Use proper tools! Removing plugs is straight forward, though using the wrong tools can cause you more difficulty. Changing plugs is probably within the privileges of the pilot/owner (it is in Canada), but satisfy yourself you do have the authority to do this work unsupervised.
Should you decide to follow ZA's advice, clean the mag lead caps at the plugs, so you can read the "T1" etc. to assure that they go back on the correct plug (lead length and position should assure this, but it's gotta be right). Hold the smaller nut on the lead (probably 7/16" wrench) while you turn the larger one (probably 3/4"). 7/8" deep socket needed for the plug. Do not drop the plug on the ground, a hard knock wrecks them. Take note of all the washers, and my be CHT lead under one plug.
Once out, and before swapping top for bottom, use a stainless steel brush (BBQ brush works) to clean the electrodes. Inspect for broken brush wires left in there before retuning the plug to the other position. Probably torque to 300 inch pounds. Really - you should use a torque wrench for that!
If you have done all of this, and a ground run satisfies you that the problem is rectified, as long as everything is done up, you're safe to fly. If that did not fix it, call your mechanic, there's nothing else you can do without direct qualified oversight.
Do not allow moisture to enter between the spark plug lead and the plug (like doing the work in the rain), you will frustrate yourself when you start the engine.