Mass update links with phpmyadmin

I have several blogs which have amazon content posted with wprobot on 1000's of posts.

I need to change all the tags in the posts like a mass find and replace so that for example

hxxp://amazon.co.uk/blahblah?tag=mysite-21

becomes

hxxp://amazon.co.uk/blahblah?tag=mysite2-21

I only want to change the tag, not the whole link.

Anyone know how to do this in php my admin or can point me in the right direction?

Thanks