How much does the weight of water change when it is frozen?

The weight of a given amount of water does not change with a change in temperature. The density does.

http://van.physics.illinois.edu/qa/listing.php?id=1745

https://en.wikipedia.org/wiki/Ice

Weight of a given mass doesn't change, so I assume you are referring to the change in density (the mass, and hence weight, of a given volume of a substance). Liquid water's density changes with temperature, so "when it is frozen' must be specified to refer to the change in density between liquid water at 0 degrees and solid ice at 0 degrees. There are several forms of ice, not all with the same density, so only the common hexagonal crystalline form will be considered for simplicity.

"An unusual property of ice frozen at a pressure of one atmosphere is that the solid is some 9% less dense than liquid water. Ice is the only known non-metallic substance to expand when it freezes. Ice has a density of 0.9167 g/cm³ at 0 °C, whereas water has a density of 0.9998 g/cm³ at the same temperature. Liquid water is densest, essentially 1.00 g/cm³, at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the temperature drops to 0 °C."

https://en.wikipedia.org/wiki/Ice