As
pdurrant has already pointed out, you can't edit or compile dictionaries with Calibre.
Updating a dictionary involves the following steps:
1. Unpack the dictionary with
KindleUnpack or the
KindleUnpack Calibre plugin.
2. Edit the dictionary HTML source file with an editor that can handle large text files.
3. Compile the dictionary with
KindleGen or
KindlePreviewer.
You also might want to have a look at the dictionary section of the
Kindle Publishing Guidelines.