how do u insert text into a specific place in a file, say the middle for example, without changing the name for that file

Inserting text into a file either through standard UNIX commands such as sed(1) or awk(1) or through use of a customized program will require, at least, a temporary file. An example of the former would be:

FILE=filename
sed -e "s/this text/for that/" 1>/tmp/$FILE.tmp && cat /tmp/$FILE.tmp > $FILE && rm /tmp/$FILE.tmp

A C example is a bit more complex (and will not be attempted without additional specifications) but would roughly work as:

1 ) Open current file
2 ) Open temporary file from tmpnam(3C)
3 ) Read current file making changes as necessary and write to temporary
4 ) Rewind both files
5 ) Read temporary file writing to current file
6 ) Resize/truncate current file (Necessary if temporary file is smaller than original, not required otherwise; good practice to always do anyway)
7 ) Close both files

I hope this is helpful.