SED - Change - Change string on a specific line number

Replace the string on a specific line number only. 

sed '3 s/unix/linux/' filename

Example

Input file

test.txt
aaaaa
bbbbb
ccccc    
ddddd
eeeee
aaaaa
bbbbb
ccccc    
ddddd
eeeee

Run: 

sed '3 s/cc/linux/' file.txt

returns: 

aaaaa
bbbbb
linuxccc    
ddddd
eeeee
aaaaa
bbbbb
ccccc    
ddddd
eeeee