WolframLang: Change Element in List

ReplacePart

Replace by Pattern ReplacePart

"replace second element by x"
ReplacePart[ {a, b, c}, 2 -> x ] === {a, x, c}

"replace second element by x"
ReplacePart[ {a, {3, 4}, c}, 2 -> x ] === {a, x, c}

"replace multiple positions"
ReplacePart[ {a, b, c, d}, {2 -> x, 3 -> y} ] === {a, x, y, d}

"replace element at position {2,1} by x"
ReplacePart[ {a, {3, 4}, c}, {2,1} -> x ] === {a, {x, 4}, c}

you can also modify a element by assignment.

var[[n]] = val

Assign a new value at index n in var.
note, expr[[n]] is shortcut for Part Part

xx = {a, b, c}
xx[[2]] = 4
xx
(* {a, 4, c} *)