I have a fixed width file and i need to change 36th field to "G" in for about random 20 records? How can I do it?

Since I'm not sure what exactly you mean, here's a list of sed-one-liners, I hope you'll find what you looking for.

sed '4,6 s/\(.\{35\}\)./\1G/' temp.txt>sample.txt

This will modify the 36th character from line number 4-6 (u can modify it for 20 lines)

hope this helps

can you let me know where i should give the character to what it needs to be modifed too.

sed '4,6 s/\(.\{35\}\)./\1G/' temp.txt>sample.txt

change the value of G to the one u want