Remember that velocity is a vector so both magnitude and direction matter.

So, lets say the downwards direction is negative, tht means that the magnitude and direction of the ball when falling is $-1v$ and $0.75v$ when going upwards.

The mass $m$ is constant.

Becusae the upward and dowward directions are opposites, it means that to get the change in momentum we subtract the two, so:

\[ (0.75v \times m) - (-1v \times m)\]

which yields $1.75mv$ upwards.