We installed a brand new Reliance Electric Hot Water Heater, but no hot water, only lukewarm! Replaced top heating element and still no hot water!! Any suggestions as to what we need to do now?

If you have some hot water from dual-element water heater, then that tells us:
Water heater is 240Volt.
Upper element is working. If upper element is not working, then heater has NO hot water.
Reset button is not tripped.
Electricity and breaker to water heater are delivering 240Volts to heater.

Remaining suspects are:
1) Bad upper or lower thermostat: Solution: replace both thermostats for 28$.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
2) Bad lower element: Solution test and replace element:
http://waterheatertimer.org/How-to-test-water-heater-element.html
http://waterheatertimer.org/How-to-replace-water-heater-element.html
3) Loose wire or loose connection: Solution: open both covers and inspect for burning. Tighten all screws very tight.
4) Bad wire: Solution: Buy cheap multimeter from Home Center and do a 30 minute troubleshoot of entire water heater before replacing any parts:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
When testing for voltage, rotate multimeter dial to 240-250-277VAC
When testing elements or continuity, rotate multimeter dial to Ohms.