Hi Sinan. I'm Greg, an installation specialist, 10 year Windows MVP, and Volunteer Moderator here to help you.
I also had Office 2010 and replaced it with Office 365 but changed the defaults before I uninstalled 2010, so I can guide you here.
One thing we find out is that Office 365 doesn't group the apps under Office 365 but puts them under the name of the Office app. So keep that in mind.
Go into Settings > Apps > Default Apps > Choose Default by App, then select, Word, Excel, Outlook, Powerpoint, Publisher, etc for all 365 apps you have installed, then Manage button which will show you all file types related to each of those apps so you can make sure the App is chosen for all of those listed that you want.
Another way is to run an Office 365 repair as shown here to reinstate the defaults:
https://www.thewindowsclub.com/install-reinstal...
I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I solved your problem, I will keep working with you until it's resolved.
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