ReplacePart

ReplacePart[expr,inew]

yields an expression in which the i part of expr is replaced by new.

ReplacePart[expr,{i₁new₁,i₂new₂,…}]

replaces parts at positions i_n by new_n.

ReplacePart[expr,{i,j,…}new]

replaces the part at position {i,j,…}.

ReplacePart[expr,{{i₁,j₁,…}new₁,…}]

replaces parts at positions {i_n,j_n,…} by new_n.

ReplacePart[expr,{{i₁,j₁,…},…}new]

replaces all parts at positions {i_n,j_n,…} by new.

represents an operator form of ReplacePart that can be applied to an expression.

Details and Options

Explicit negative part numbers count from the end. »
Part position specifications can be patterns.
Each pattern is effectively tested against each list of part numbers for parts in expr.
Patterns p (such as x_) that are not explicitly lists are treated as {p}.
Patterns can include constructs such as __, representing position specifications of variable lengths.
ReplacePart[expr,i:>new] can be used to replace a part without first evaluating it. With a rule such as patt:>new, new is evaluated separately for each position that matches patt.
With the default option setting Heads->Automatic, a head is replaced only when the corresponding position specification is explicitly 0.
With Heads->True a head is replaced whenever the corresponding position specification matches 0.
Heads->False never replaces heads.
ReplacePart can be used on SparseArray objects.
ReplacePart[i->new][expr] is equivalent to ReplacePart[expr,i->new].