If you are arriving at an "A" gate at FRA (from outside the EU, like on UA58 from SFO), you will actually be forced to walk upstairs (just after leaving the jet bridge) and will enter the terminal among the "Z" gates. From there, follow the signs to the "A" gates. This will involve both going down one level and clearing passport control to enter the EU.
If you arrive at a bus gate, just follow the signs to the "A" gates. This may involve a tunnel and long walkways, but just keep following the signs.
You have enough time if your first flight is mostly on-time and you keep moving once you get there! Good luck! Please let us know how it goes.
JK-SFO