How to extend memory?

It is possible, You need to solder to SD card(or uSD adapter) directly to motherboard. It's difficult, and if you are a rookie in electronics, better don't try it.

There are a lot of videos and web pages out there that show TomTom ONE owners how to do this. Just make sure that the TomTom ONE model you have is the same as the one in the video, for example, one video is based on a ONE XL, and the ONE 130 (30 series) circuit boards are the same...I believe. But if you have a different model, make sure you research it thoroughly before tackling any of this...and the pin connections are smaller than the size of the head of a pin, so you're going to need a lot of lighting and a magnifying glass on a stand, plus a very small pencil soldering iron...if you use a 100W soldering gun, you're going to burn up the board and get tons of solder all over everything, thus you unit is now trashed. Good Luck!

If you add more memory to a tomtom XL 335 LM (joke on lifetime maps…), and then go to TomTom Home and do an update, will TomTom actually allow you to download/install larger maps on this discontinued device?