First, install Notepad++.
Next, download the latest HexEditor .ddl plugin. I downloaded HexEditor_0_9_5_UNI_dll.zip. (For some reason, the most obvious green download button takes you to an HTMLTag_plugin, which looks like a dead-end to me.)
Finally, open the zip file, select the HexEditor.dll file and place it in the folder
C:\Program Files (x86)\Notepad++\plugins\HexEditor
if you installed the 32-bit version of Notepad++ and
if you installed the 64-bit version. (For older versions of Notepad++, omit the HexEditor folder)
To test if your install was successful, restart Notepad++, and you should see a menu option Plugins -> Hex-Editor -> View in Hex. This will toggle whether you are viewing the file in hex.
The Notepad++ hex editor does not have undo support, so I no longer use it. (I am still using Notepad++ itself.) Instead, I use HxD, which is both powerful and beginner-friendly.
Thanks to Bálint and momolang momo for pointing out the need for the HexEditor folder in newer versions.