FlattenAt

FlattenAt[list, n] flattens out a sublist that appears as the nth element of list.

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

Examples

Flatten at position 2:

FlattenAt[{a, {b, c}, d}, 2]
(* {a, b, c, d} *)

Flatten at nested position:

FlattenAt[{{a, {b, c}}, d}, {1, 2}]
(* {{a, b, c}, d} *)

Please visit the official Wolfram Language Reference for more details.