Well, it's something difficult for me to write everything here related to field type update :). let me try to give you some pointer.
While creating a field type, you were creating a $info array having "name" and "version". If you are going to update you field type, first step will be to higher the "version" value within the $info array.
Second thing: the field type file will have a function update like:
function update($from)
{
if($from < 2.0){
[process the stuffs which you need to update in the updated version of 2.0 in the case your previous version was less than 2.0]
}
}
Now it's depend on you, what you would like to update. If you would like to update the settings or field type display then you will need to modify the code within their function. In the update function you can re-structure database related stuffs.
I hope, my little pointer will help you.
Best Regards,