CakePHP lets you override pretty much anything, you just need to understand its structure. The best thing to do is to copy the default.ctp file from your cake folder in APP_DIR\cake\libs\view\layouts\default.ctp
.
Once you've copied this, place it in APP_DIR\app\views\layouts\
and cake will start using that file. To customize it, simply open up that file and change it in your editor.