ReplaceAt

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

ReplaceAt[expr,rules,{i,j,…}] replaces the part of expr at position {i,j,…}.

ReplaceAt[expr,rules,{{i1,j1,…},{i2,j2,…},…}] replaces parts at several positions.

ReplaceAt[rules,pos] represents an operator form of ReplaceAt that can be applied to an expression.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.