ReplaceAt

ReplaceAt[expr, rules, n] transforms expr by replacing the nth element using rules.

• ReplaceAt[expr, rules, {i, j, ...}] replaces the part at position {i, j, ...}.
• ReplaceAt[expr, rules, {{i1, j1, ...}, {i2, j2, ...}, ...}] replaces parts at several positions.
• ReplaceAt[rules, pos] represents an operator form that can be applied to an expression.

Examples

ReplaceAt[{a, b, c}, x_ :> x^2, 2]

ReplaceAt[{{1, 2}, {3, 4}}, x_ :> 10 x, {2, 1}]

ReplaceAt[x_ :> -x, 1][{a, b, c}]

*See the official Wolfram Language Reference for more details.