I have old kindle PC, DRM removal is quite easy with calibre but for some books they're asking me to update kindle and I guess we cannot strip drm... Can anyone help?

[Dendra]

I had to downgrade the Kindle app on my computer to be able to strip DRM. I think it was 1.4 something?

[Beetzart]

Yes you better use an older version of the Kindle app.
see here for more info:

https://www.mobileread.com/forums/showthread.php?t=283371

Version 4.23 or older works fine.

The issue isn't calibre. Kindle has changed the format of the digital books and the plugin that pretty much everyone uses doesn't work in conjunction with later versions of caliber.