If you’re unable to get the automatic upgrade for Windows 10 to work with your Dell Venue 8 Pro, try using the Microsoft media creation tool to get there. Depending on how much free space you have, an SD card may be required for the download to be stored. The media creation tool will prompt you if you’re short on space for this operation.
For my upgrade, I opted to have the media creation tool and Windows 10 setup wipe everything so I could start fresh. Since all of my important files, pictures, etc. are sync’d online, I wasn’t concerned about the loss of any local content. This is probably not the case for most people, so back up your files before you start. Note that although this was written with the Dell Venue 8 Pro in mind, this process works on an activated Windows 8.1 setup as well. The key is it must be an activated instance of the operating system, or the tool will prompt you for the appropriate product key.