Editing executable files (EXE, DLL, etc.) with a hex editor is tricky.
If you are editing a string in a file, make sure that you are just
overwriting data (do not insert or delete any bytes from the file, just
overwrite values). If you want to shorten a string, sometimes you can
overwrite data at the end of the string with '00' bytes and everything
should work fine.
However, if you need to delete or insert bytes from the
file, you have to update your EXE file in a number of other places
in order for the EXE to be a valid executable again. You may be
able to do this by using an EXE template, as available from our
online repository. If you are doing something complex, it probably
would be better to use a tool designed specifically for editing
EXE files. These tools are called 'PE editors' (PE stands
for Portable Executable) and can be located with a search engine
such as Google.