Check to see if you have the old owner's manual for the heater first. It will tell you how to increase the temperature. If you don't have the manual, and it's a gas unit, follow the copper tubing to the small "box" at the base of the heater. There is/was a temperature-setting dial on the face or side of this "box". If your heater is electric, look to where the wire enters the heater (usually at the top of the unit). There will be small rectangular plate (with one or two screws) on the side or top of the heater near this location. You may want to inspect the outside of the heater first for a temperature adjusting mechanism.
First, please turn the power off to the electric unit at the circuit breaker box before doing anything. Remove screws, plate, and push the insulation to the side. Depending on the unit, you will see wires attached to a black distribution box. There may be a small dial with a screwdriver slot at this distribution box. The dial will have a W<>C, H<>C, or some reference to temperature. Increase the temperature in small increments, replace the insulation, plate, and screw. If you don't find this W<>C adjustment, look for a plate on the outside of the unit below the upper access plate. When you are done turn the power on and check the hot water temperature later.
If it's a really old heater, just add more wood to the fire
If it's an older heater and your water supply is mineral rich, I'd personally leave the heater settings alone. The interior of a 20 year old tank may be pitted. By increasing the tank temperature, it may aggravate any pitting in the holding tank. You just may want to fill a cooler with hot water from your water heater and then add hotter (boiling?) water from the stove. Then add your gear and adjust the temperature to last the recommended temperature and time.