You will need to know HEX editing before you can actually use this function in Notepad++. Luckily, hex can be learned through tutorials found through Google Search.
To enable HEX editing, start Notepad++ and go to Plugins > Show Plugin Manager. Locate Hex Editor in the list and check it, then press Install and restart Notepad++ when require. You can start the plug-in using Plugins > HEX Editor.