ReplacePart[expr, new, n] yields an expression in which the n part of expr is replaced by new.
ReplacePart[expr, new, i, j, ... ] replaces the part at position i, j, ... .
ReplacePart[expr, new, , , ... , , , ... , ... ] replaces parts at several positions by new.
ReplacePart[expr, new, pos, npos] replaces parts at positions pos in expr by parts at positions npos in new.
Example: ReplacePart[a, b, c, d, x, 3] .
The list of positions used by ReplacePart is in the same form as is returned by the function Position.
ReplacePart[expr, Hold[new], pos, 1] can be used to replace a part without evaluating it.
If pos and npos both specify multiple parts, each part in pos is replaced by the corresponding part in npos.
See The Mathematica Book: Section 1.8.6 and Section 2.1.4.
See also: Part, Extract, MapAt, FlattenAt, Insert, Delete, Sequence, StringReplacePart.