But how does it change that constant velocity?

但是力是如何改变速度的呢？

So now we're dealing with a scenario where the block has a constant velocity.

假设物块向下具有不变的速度假设物块向下具有不变的速度。

Or you actually could say an object with constant velocity will stay having a constant velocity

具有恒定速度的物体将一直以这一恒定速度运动，直到有一合力作用于这一物体上。