sed command FAQ: How can I use the Unix/Linux sed command to edit (modify) files in place?
The short answer is that you just need to use the -i or --in-place sed arguments, as shown in the sed man page:
-i[SUFFIX], --in-place[=SUFFIX] edit files in place (makes backup if extension supplied)
Then, if you have a file named 'hello.txt' with contents like this:
jello, world this is a test
you can then run a sed command like this to modify that file:
sed -i -e 's/hello/jello/' hello.txt
That simple sed command changes the first 'hello' on each line to 'jello'. Not a great example, but simple enough.
Notice that this sed command with the -i option overwrites your original hello.txt file, so of course you want to be very careful with this command, and make backups of your original files, and move them to other directories before issuing this command.
As you can see from the sed man page, you can also provide a file suffix, and sed will make a backup of your original file, using the suffix you provide. So a command like this:
sed -i.bak -e 's/hello/jello/' hello
will create a backup file named hello.bak, which will contain your original file contents. Again, I strongly encourage you to make a backup of your files somewhere else, because when sed commands go wrong, they can wreak a lot of havoc.