how to upgrade my system

Nobody knows what kind of upgrade you want to do. Nevertheless, you won't lose documents that are backed up to a destination outside your computer, like a portable hard drive or a USB flash drive.

First of all, upload all the folders and files in OneDrive (make sure you remember the mail ID and password of your OneDrive account)

Once done, go to Settings -> Windows Update -> Check for updates

Then update it to the latest version that your system supports.After the update, log in with the same mail ID and password, and your data will be as usual in the Documents folder, and all folders where you did the backup in OneDrive.