how to write finding output to same file using awk command

Not possible per se. You need a second temporary file because you can’t read and overwrite the same file. Something like:

awk '(PROGRAM)' testfile.txt > testfile.tmp && mv testfile.tmp testfile.txt

The mktemp program is useful for generating unique temporary file names.

There are some hacks for avoiding a temporary file, but they rely mostly on caching and read buffers and quickly get unstable for larger files.