Those two options given by Jaygiven and Grumpy are pretty much the only options you have.
Here's a video on how to install a water heater thermostat:
Or, as Jaygiven said, just slightly open the water heater bypass on your heater. This will allow some cold water past the water heater and it will mix in with the hot water lines. This is a trial and error exercise. Keep adjusting until the temperature is what you desire.
On the other hand, having the water be very hot can help with the shower. Because you will be cooling the water down at the shower faucet, you will be using less hot water and some cold water and your hot showers can last longer before you use up all the hot water in the hot water tank. Personally, we take military showers and using up the hot water is NEVER an issue for us, but for some folks that have full hookups and don't worry about water conservation, they can run out of hot water during their shower, as the HW tank is usually only about 8 gallons. That is way more water than we ever use to take a shower.