Algorithm

$1.$ Run DFS to compute tin[v] and tout[v] for all nodes.

$2.$ Build array arr where arr[tin[v]] = value[v].

$3.$ Build segment tree on arr supporting range sum and point update.

$4.$ For subtree sum query on $v$: query segment tree on range [tin[v], tout[v]].

$5.$ For value update on $v$: update segment tree at position tin[v]. Each query takes $O(\log n)$.

Space complexity is $O(n)$ for the data structures used.