Please use code tags for code snippet and quote tags for text input/output.

Example i gave is for file and it refers to line such as first , exclude last line.

Code:

sed -e '1{n};$!s/./&I/10' line

is wrong here.

Is below are contents of file or file names ?

Here is what i can simulate with multiple files:

Code:

ls 
123.txt  456.txt  f1  f2

Code:

for file in *.txt
do
echo "File Name=" $file
sed '1{n};$!s/./&a/10' $file
done

Gives output :
Once you are sure that this is what you want, you can use in-place edit option

Code:

-i

to update in the file itself. Please note , you can not undo .