How to edit a .odt file from the terminal?

The closest thing to what you're looking for is wordgrinder, a terminal-based word processor.

You can install it executing:

sudo apt install wordgrinder

Here is the official website of the project: https://cowlark.com/wordgrinder/

This is the Github repo: https://github.com/davidgiven/wordgrinder

And a quick Survival Guide: https://gist.github.com/davidgiven/1aba97e96a9b8f9b573b

If you check the Importing and Exporting sections you will find:

Importing

Imports basic content from ODT files. OpenDocument is complicated and hard to parse but WordGrinder will do its best to apply appropriate styles. Unsupported features are ignored.

Exporting

This produces an ODT file which can be read by LibreOffice and a variety of other major word processors. Character and paragraph styles are exported and are mapped to OpenDocument styles called P, H1, H2 etc.

libreoffice comes with an option to convert an odt file to plain text, which then can be edited with your preferred command-line text editor and (if wanted) converted back to odt.

1. Convert document.odt:

   libreoffice --convert-to txt document.odt
   

2. Edit the file with your preferred text editor, e.g.:

   gedit document.txt
   nano document.txt
   vim document.txt
   

3. Convert it back. The --convert-to takes a filename extension as an argument, but you may also specify a filename suffix to prevent libreoffice from overwriting the original file (which it does without asking!):

   libreoffice --convert-to _new.odt document.txt
   

For the conversion there's also odt2txt, I'd just try both and compare the results.

If you're using vim you can configure it to automatically do the conversion for you in the background, see: Is it possible to easily work with .odt, .doc, .docx, .rtf, and other non-plain-text formats in Vim? The answer there uses odt2txt for the conversion, but it should be possible with libreoffice --convert-to as well.