FlattenAt[list, n] flattens out a sublist that appears as the n 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, , , ... , , , ... , ... ] flattens out parts of expr at several positions.
Example: FlattenAt[a, b, c, d, e, 2] .
See The Mathematica Book: Section 1.8.14 and Section 2.2.10.
See also: DeleteCases, Flatten, Sequence, SlotSequence.