Explain when and how tensions between the superpowers decreased and when and how they increased

Assuming you mean the U.S and U.S.S.R, then tensions started decreasing in the 70s, partly because of the Vietnam war being seen as a product of the Cold War and the war being very unpopular. Additionally, the U.S.S.R wasn't doing very well internally, and the reforms Nikita Kruschev institued in the U.S.S.R made it hard to convince the people that they were doing better than the U.S, as the press began covering things differently. By the time the Berlin Wall came down, the Cold War was basically over.
Tensions started around the late 40s, and peaked in the 50s and around the Bay of Pigs. This happened because the two governmental systems and political ideologies contrasted, and the two nations despised each other as a result.