The US has an official building in Tel Aviv and another in Jerusalem.
The process is simple: Take a screw driver, walk up to the building in Tel Aviv, undo the screws holding the building's "Embassy" placard and remove the placard. Drive to Jerusalem, go up to the building, drill pilot holes, screw the "Embassy" placard to the building.
Optional (but recommended) step:
Fill in the empty screw holes in the Tel Aviv building with cement (adds a step to switch the Embassy back).
Interestingly, most of the countries that have a diplomatic presence in Jerusalem do not consider these buildings to be their embassies or consulates in Israel. Rather, they established them as diplomatic missions to Jerusalem as a separate legal entity under the 1947 United Nations Partition Plan for Palestine.
It's also worth noting that the United States has never officially changed its position regarding the legal status of Jerusalem under the 1947 partition plan.