str="I want to abc replace a word abc in a sentence abc of file abc by giving abc specifying a abc multiple abc range to abc the sed abc command abc."

str="I want to abc replace a word xyz in a sentence xyz of file xyz by giving abc specifying a xyz multiple xyz range to xyz the sed abc command abc."

( (2<=NR && NR<=4) || (8<=NR && NR<=10) || (12<=NR && NR<=15) || (18<=NR && NR<=20) || (26<=NR && NR<=29) || (...) )

sed '-es/abc/xyz/'{8..6} '-es/abc/xyz/'{4..2} <<<"$str"

sed '-es/abc/xyz/'{8,6,4,2} <<<"$str"