hello, a friend of mine sent me a file which consist extension name anki2 collection.anki2 like this
and I can open this file with my laptop and all the other files extension name is apkg. thus, I also changed extension name but it’s still not working and he said he can open this file in his laptop. and asked me if I updated with latest version and I did and tried again. and still it is not working. so, if there is anyone who knows how to solve this problem. can you answer me? 
He didn’t export the cards, but sent you his collection file. Create a new profile, put the file instead of the new profile’s collection.anki2 (do not confuse with your ordinary profile — make a backup), export from there (decide if you want to export the scheduling information) and import into your ordinary profile.
it’s too hard to understand. could you explain more easily?
Everything but the media files is stored in a collection.anki2 file in the profile directory, but to transfer stuff between different collections (profiles), it is usually better to export and import (Anki’s own format is apkg).
Backups: https://docs.ankiweb.net/#/files?id=backups
Profiles: https://docs.ankiweb.net/#/profiles
File locations: https://docs.ankiweb.net/#/files?id=file-locations
Exporting: https://docs.ankiweb.net/#/exporting
Importing: https://docs.ankiweb.net/#/importing
sorry for disturbing you too much. but could you explain in the way, what I have to click and what I have to move to where like this?
You could instead ask your friend to use the File > Export function in Anki to export the decks he wants to share with you.
That’s what I’d recommend, as it will also remove their remove history, which you probably don’t want.