Place your upper back on a Stability ball and your feet flat on the ground, with the legs opened to provide stability.
Hold a Dumbbell with both hands and extend your arms up to a vertical position.
Let the weight drop down slowly overhead, as low as your flexibility allows.
Then bring the weight back up.
Form tips
Do not let your back arch too much (a little curvature is ok); focus on opening at the shoulders instead ( Shoulder flexion backward ).
Approach the bottom position slowly to avoid straining your shoulders. Once they are well warmed up, aim to go a little bit lower to gain flexibility in that position, while strengthening it thanks to the weight.