How to sort file I/O and update existing file

If the text file is small, you can read each line into memory, apply a sort on the lines, then write them all back to the same file in their sorted order.

If a large text file, you can sort the lines in the text file without loading more than 2 into memory at once. Start with the first line, read it in, and call this the "current" line. Next, compare it to the second line in the file. If they are not in the correct order, swap them, then use the previous line in the text file as the "current" line (if you reach the beginning of the file, just reapply the same algorithm starting from the first line in the file). If the lines match, move on to the next line in the file and make that the "current" line. Continue this procedure until you've reached the end of the file.

There are faster ways of sorting text files, but that is the simplest way I could describe it to you.