Its in your public_html folder.
You can use cpanel > file manager to edit the file
or if using FTP, make sure you can see hidden files.
Insert the code anywhere in the file.
In the rare event you do not have one, create a text file named .htaccess
I prefer to copy the file using FTP to my computer and edit it and then upload it back to my space.
That way I have a quick backup.