1. With any major software.hardware change/upgrade, "losing files" is always a possibility. Power gets interrupted, someone clicks the wrong button....
Always always have a backup, and know how to recover it.
2. You can 'probably' still Upgrade to Win 10 for free. Assuning it is currently Win 7 Home, Win 10 Home is what it will upgrade to.
Backup your data.
Discover all your existing software license keys, and have them documented. Belarc Advisor can do this.
GO here and create a WIn 10 USB to install with:
https://www.microsoft.com/en-us/software-download/windows10
You'll need a blank 8GB USB flash drive
Boot from that, and it will probably give you the option to Upgrade the current Win 7 to Win 10.
But first, backup your data...