Did they make a blok file or just use HTML?

Go to your Nuke/Admin/Blocks. Click on the block and then look at whether it is a block file or HTML.
If it is a block file, you will need to get the blocks/block-filename.php file off of your server via FTP and edit it in a PHP editor.
Otherwise, if it is a standard HTML block, copy the html to an editor and edit it then copy/paste the changed HTML back.