What is the best way of editing .mht(web archive) files?

An MHTML file is a web page archive format. It is meant to be stored and viewed but not to be edited directly.

However, you can easily extract the MHTML file to a regular HTML document (with linked files), edit it with your favorite HTML editor and then export it back to an MHTML archive (including the linked files).

Since you're using Internet Explorer, note that you can open/save between HTML and MHTML files. This can effectively be used to unpack, edit and repack the MHTML archive. Google Chrome can do this as well.

You may also find software that are able to edit the MHTML file directly (doing the unpacking/repacking in the background). Microsoft Word seems to be able to do this, but depending on your document structure, it may impact the content layout.

A quick look at the wikipedia entry for MHTML shows that it's an archive format, a little bit like a zip or rar archive. In order to edit a .mht you will need to unpack it, edit the required file then repack the archive.

You don't say what platform/software you are using but if you do a websearch for ".mht unpacker" you should be able to find something to do the job.