The trusty vim can be used as a hex editor by using the xxd filter. Just use the following command:
:%!xxd
This basically means:
%!: select all xxd: filter
Once the filter has been applied the view can be something like this (for a .png file):
If changes are needed, they should be made to the left (binary) side. Changes to the right side will be discarded.
To go back to the normal view apply the filter in reverse:
:%!xxd -r
That’s all there is to it, vim as hex editor. Enjoy!