See http://www.nokia.co.uk/softwareupdate for more info.

However, I've just tried to update my E70 and the computer lost
connection with the phone near the end of the update, at which point the
screen had 'test' and other such words in different sizes of writing.
The computer said to remove the battery, which I did, but on replacing
it the phone no longer works and continually reboots itself (the word
NOKIA appears, then flashes, then it looks like it reboots).

And whilst reading this, this error has just appeared on the computer
screen:

"Nokia Service Layer Host Process has encountered a problem and needs to
close"

Use the update at your peril. If anyone from Nokia is reading this,
perhaps they would like to suggest what I should do now? You can remove
nospam from my e-mail address to make it work.

Mark.