Is there any free or cheap editor for Windows, that would allow to edit files with .woff extension (I checked in css that the font file has woff extension). These are fonts offered by Google Webfonts and they are free to embed, modify etc. I'd like to only add diacritic accents to some letters for specific language and embed the font on a website.

There's a free, cross-platform tool called FontForge, which can be used to create, edit, and view a wide array of font types.

Homepage: http://fontforge.org/
Github: https://github.com/fontforge/fontforge/

Here are some screenshots of it in action:

WOFF

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WOFF2 <sup>(added in 2018)</sup>

Woff2 Character Map

Editing Characters

Editing Characters