If your device isn't running Windows 10, refer to the Windows 8 instructions.
For more info regarding OneDrive, such as how to copy, move, save, view and share files, refer to Microsoft's support article.
Windows® can automatically back up device content to OneDrive including settings, app list, photos, videos, and messages.
To utilize the backup functionality, the device must be signed in with a Microsoft account and there must be sufficient space in OneDrive to store the data.
Open File Explorer.
From the Home screen, navigate: All apps icon (located in the upper-left) > Windows System > File Explorer.
Locate the file(s) you'd like to add then drag and drop into a desired OneDrive folder (located in the left-pane), then tap or click Copy here.
File Explorer icons show you the sync status of your offline folders and files: