It will update when it's ready....which is not very often......but.....if you dont keep an eye on it, the update to next version will happen without you even noticing !
follow my directions to "lock" it....that stops even the Update Manager from updating it.
If it is staying at 6.4....and you want it to stay there....leave it alone.....what's not to like ?
Although, i must admit it is rather strange that it has not updated libreoffice ...
If it gets sneaky and updates it without yiou realizing....go to the link below...it is the Archive for Libre Office....you will need to scroll down a fair way....and select the exact 6.4.x.x ...that you are using
You may have to uninstall the libre Office that it has updated to
From memory i think you only wanted writer and calc??........you can download those individually or you can simply download LibreOffice ....which will give you whole lot.