Word

[manukarik]

https://www.google.co.uk/docs/about/

Google Docs for work costs $12 a month but the ordinary version of Docs is free with 15 GB per user Cloud storage so go for that one. The Docs save themselves so you don't have to remember to do that. You can also save in Word .docx format:

Transform your Google Docs to Microsoft Word easily! To do this, first open up the document you want to convert. Then click the “File” tab at the top left corner and scroll down to select “Download.” Choose the “Microsoft Word (. docx)” file format and save the document on your hard drive.

[NickDub]

To try Google Docs set yourself up with a gmail account - the rest then follows automatically.

[Ian Nelson]

Within the depths of Windows 10 you will find, Notepad, very very basic, and also you should find Wordpad, a simplified version of Word, but still a word processor,adequate for your letter-writing needs.  I checked the Microsoft website and it appears you can re-install Word or download it from their site.
I've used LibreOffice suite for years  (free) and it's word processor is called Writer ... equivalent to Ms Word.
However, I frequently Download Song Lyrics and it has recently become the way to offer them as PDF (portable document format ) files and if locked then you can't EDIT them easily, except... by using Editpad Lite (free) which gives you the power to do what you like with almost any Text file.
If you are only doing letters as correspondence then I recommend Wordpad.
cheers, Ian