Problem - Range Sum Query 2D Mutable

Given a 2D matrix, implement:

• update(row, col, val): Update matrix[row][col] to val
• sumRegion(row1, col1, row2, col2): Return sum of rectangle

Example:

matrix = [[3,0,1,4,2],
          [5,6,3,2,1],
          [1,2,0,1,5],
          [4,1,0,1,7],
          [1,0,3,0,5]]
sumRegion(2,1,4,3) → 8
update(3,2,2)
sumRegion(2,1,4,3) → 10

This requires either 2D segment tree or 2D BIT (Binary Indexed Tree).

Constraints: matrix up to $200 \times 200$, up to $10^4$ operations.