print lines 3 through 7 of a file: sed -n '3,7p' somefile | sed -i '4s/xxx/bbb/' f1

print lines 3 through 7 of a file: sed -n '3,7p' somefile

Trim leading and trailing whitespace
sed -i 's/^[ \t]*//;s/[ \t]*$//' somefile
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sed -i 'Ns/.*/replacement-line/' file.txt

where N should be replaced by your target line number. This replaces the line in the original file. To save the changed text in a different file, drop the -i option:

sed 'Ns/.*/replacement-line/' file.txt > new_file.txt

centos@192 Documents]$ sed -i '2s/aaa/bbb/' f1
[centos@192 Documents]$ cat f1
aaa
bbb
aaa
bbb
aaa