What if instead of dying in April of 1945, Roosevelt lives until the end of the year and sees the atomic bomb successfully tested. How would he choose to use it?
Not much. In the last month of his life, Roosevelt had already figured out that Stalin wasn't really a guy he could do business with like he thought he could but it was already too late to reverse course on issues like Eastern Europe and Japan, especially given public attitudes and everything else. He might make some half-hearted attempts to bring Stalin into line at Potsdam (or it's ITTL equivalent), but there just isn't anything he truly has to threaten or offer Stalin that isn't a giant bluff Stalin can easily call.