Stand alone water heaters have temperature controls in different locations.
Most residential electric water heaters operate on 240 volts. Please, shut off power first whenever you need open or remove panels for adjustments or servicing.
"Traditional" electric water heaters have them behind removable panels - like the top and bottom gray panel on the water heaters on the right side of the photo. Both panels have adjustable controls behind them and both should be adjusted to the same setting.
Some newer electric water heaters use electronic controls. These may be accessible without removing panels and even have digital displays. Others have controls that are located on the top of the heater - behind a panel. Electronically controlled water heaters may or may not need to be powered on when making adjustments. Check your manual if in doubt.
Gas (propane & natural gas) type water heaters have the temperature control on the gas valve. There are often two controls on the valve, one for the pilot (off, light and on positions) and a second for temperature adjustment.
A tankless water heater built into your home's furnace is controlled by an "aquastat". An aquastat is a fancy term for a thermostat to control water temperature. A typical aquastat is mounted directly on or very near the furnace and looks like this:
Inside, are several temperature settings. Here's a photo of the interior of the Honeywell aquastat:
The Hi dial should never be set to a value lower than the Lo dial. "Safe" settings are Hi = 180, Lo = 160 and Diferential (above "Lo") = 10. You'll need to tweak these up a bit to make water hotter. You should read more about settings for these aquastats here
. Skip down to the section titled "What Settings To Use on a Heating Boiler Aquastat Combination Control"
I hope this helps & good luck!