HI NourinE ... and welcome to the forums ...
Yes, you can go back to Windows 8.1 after updating to Windows 10. If you keep a system image of your Windows 8.1 system, that will do. By upgrading and trying out Windows 10 before the July 29th deadline, you qualify for "digital entitlement" to Windows 10 on that hardware - indefinitely (much like you would for a "product key"... the information stays on the Microsoft Windows Activation servers). If you decide you want to return to Windows 8.1 within 30 days of installing it, you can return to Windows 8.1 with just a few clicks of your mouse (no need to use the system image, unless you want to). You would:
1) Click on the Windows 10 Start Menu icon
2) Select Settings
3) Select Updates and security
4) Select Recovery
5) Select "Go Back to Windows 8.1"
Windows 10 keeps your Windows 8.1 files in a folder named "Windows.old" for 30 days - that's why it's so easy to return to it during that first month. If you decide to go back to Windows 8.1 after those 30 days have gone by, you can always use your backup system image.
Also: if the "upgrade" method doesn't handle things well, there is no penalty for trying a clean install of Windows 10. You'd use either your Microsoft account & your "digital entitlement" (if you've already tried the "upgrade" route).... or you'd enter your Windows 8.1 product key (if you decide on a clean install before July 29).
Hope things go well.