When one observation changes from wrong value 'w' to correct value 'c': Corrected sum = old sum - w + c. Corrected sum of squares = old sum of squares - $w^2$ + $c^2$. Then recalculate mean and variance using the corrected sums. This is the single most common JEE statistics problem type. Steps: (1) recover original sums from given mean and variance, (2) adjust sums, (3) recompute.