Prior to making changes to your menu, you should back up your current theme.
This will allow you to quickly revert to your original menu if you do not like the look of any of the menus applied.
Applying the new menu will replace the content of your menu.css file, so any custom changes made there will be lost. Remember to backup your theme or copy the current css to a text document if you want to reimplement some changes, or look to apply your customisations to the DESIGN > CUSTOM STYLE ELEMENTS area.
If you don't like any of the new menu styles and want to revert to your original menu, if you made a backup of the theme as recommended above - you can then restore the old theme.