I am having trouble getting 240 volts to both of my hot water heater elements after buying a new thermostat. The bottom element has 240 but the top one only is 100 how can I fix this?

100V or 120V at top?

100V or 120V at top?
110 volts bottom has 240 volts, water heater is set up for 240 all around

Elements work one at a time. When bottom element satisfied top comes on. Don't remember exactly how switching is done. Probably only one leg switched off. How did you take voltrage readings on top. Element to ground or element terninal to element terminal?

Look at this pic. And answer HK's question.

Sounds like You have only 1/2 power going to upper thermostat.. so new thermostat was miswired or new thermostat is defective. OR you took a bad reading! Let us know what you find