How to edit binary file on Unix systems

In vim You can type :%!xxd to turn it into a hexeditor. :%!xxd -r to go back to normal mode. xxd is shipped in a vim installation.

See here for some remarks about editing binary files with vim (boils down to :set binary to avoid trouble, use only the "R" or "r" command to change text, don't delete characters).

If You are an Emacs fan, see here for a guide on how to edit a binary file with Emacs.

You can also try ghex2 GNOME utilities. This give you the automated hex-to-ASCII on the side, as well as the various character/integer decodes at the bottom.
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There are much more hexeditors on Linux/Unix....

I use hexedit on Ubuntu

sudo apt-get install hexedit