This requires a fairly good understanding of isothermal processes. I am assuming the solid/liquid is already at its phase change temperature.
When a system is undergoing a phase change, it's an isothermal process,
it's said that its entropy is generally constant until it's complete.
In more practical terms (since the above assumes this is a reversible process),
Hence, for melting that mass of ice, the entropy change would be positive since liquid will be more "disorganized" than the solid.
Likewise, the entropy change will be negative for the second change since the solid is more organized than the liquid phase.