Greetings.
So the question is basically the same as it's in the title.
I'd like to write a program that changes a file by rows. So to clarify it.
(i know i shouldn't use code,/code here but i would like to separate it)
So for example a text file looks like something like this:

Happy Birthday!
Happy Mother's day!
Happy New Year!
.
.
.
Happy Blablabla!

Happy Blablabla!
.
.
.
Happy New Year!
Happy Mother's day!
Happy Birthday!

Try tac command if you are on Linux.

or Try:

awk '!(A[i++]=$0); END { while(i--) print A[i] }'  file

If you are not in interested in using a command but in writing the script yourself, you can read all the lines into an array and then print the array from the highest index down to the lowest.

I would like to write this as a script so i can use it whenever i will need it in the future. But i would like to change the file's structure. I mean i would like to overwrite the file.

You could write the result to a new file and then mv that new file to the old file.

Don't understand why put ! in awk,

awk '{A[i++]=$0} END { while(i--) print A[i] }' urfile

awk  '{A[NR]=$0} END {NR++; while (NR--) print A[NR]}' urfile