MapAt

MapAt[f,expr,n] applies f to the element at position n in expr. If n is negative, the position is counted from the end. 

MapAt[f,expr,{i,j,…}] applies f to the part of expr at position {i,j,…}.

MapAt[f,expr,{{i1,j1,…},{i2,j2,…},…}] applies f to parts of expr at several positions.

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

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