FlattenAt

FlattenAt[list,n] flattens out a sublist that appears as the nth element of list. If n is negative, the position is counted from the end. 

FlattenAt[expr,{i,j,…}] flattens out the part of expr at position {i,j,…}. 

FlattenAt[expr,{{i1,j1,…},{i2,j2,…},…}] flattens out parts of expr at several positions.

FlattenAt[pos] represents an operator form of FlattenAt that can be applied to an expression.

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