find your public/html folder. find the domain your wordpress is hosted on if you are have multiple sites. if not it should be the only one. click it and then click the wp-includes folder. that is where functions.php is located. right click and and select edit code. it will open another tab in your browser with the code you can edit. when you are done make sure you click save in the top right hand corner.
functions.php is a very important document make sure to copy and paste all original text into a safe text file on your computer incase you mess it up. you can easily go back and copy and paste it back in and start over if you do not have godaddys back up service.
... a little late to the party here ...
You do not want to edit any files in the wp-includes folder. These files are part of WordPress core files and any edits you makes to files in this area will be overwritten on the next WordPress upgrade.
If you have been instructed to edit your functions.php file, that file is found in your wp-content/<theme name> folder.
The functions.php file is specific to your theme. Put the code snippets and new functionality in there.
Hope this helps.
i half agree. i have made plenty of changes to the functions.php file. Yes, if you upgrade you will loose those changes. But i also do not upgrade a single site once i have written new rules. you can also download/upload file before/after upgrade is complete.