My partner self published a book a few years ago. We would like to change the book cover and title. It is in print...
Any advice would be very welcome. It has an ISBN No. and is available at Amazon and in some libraries...

The title is linked to the ISBN number so you will need to change the ISNB.
Not sure if you have to change the ISNB if you are solely changing the cover, but that's a moot point if you are also changing the title.
Who have you self-published through?

<Added>

oops, typo, ISBN

Publishers change covers all the time for a TV tie-in, or just to match older books with a newer book which has a different look.

Naomi's right, though: the title is part of its identity, as it were, and if you change that then I think the systems would count it as a different book, which would confuse Amazon and libraries dreadfully, I should imagine.

Emma

Just to add, if oyu are changing self-publishers it would also need a new ISBN number. And if oyu are changing size of paperback, or changing from hardback to paperback it would need a new ISBN number.

<Added>

Just double checked and a different cover on it;s own would mean a different ISBN number.