First, I'd uninstall your present version of TT Home. Then reinstall with the latest version offered at Tomtom's website. Taking care that if you are running Windows 7, set TT Home compatibility to Vista. Second, I'd reset your device to factory defaults.
After this connect device and try to update to v840. If TT Home sees updated maps but does not apply it or TT Home reports that there are no updates vs. your v835 maps or TT Home offers you a discounted deal on a map that you know is covered by the Latest Map Guarantee; then you must contact TT support. They can push the map to your account, but you have to prove it is within the 30 days of purchase or gift receipt... I had to scan my receipt and email it to them. After that, they pushed the update to account, and I could update perfectly well.
If you can update, but it goes wrong technically, contact TT support.