It is very simple. Just go to the link -> Edit Profile -> My earning -> Settings -> Configure Google Adsense -> Mark Yes for having existing adsense account..
You will get the following message :
"It appears that you already have an approved Google AdSense Account associated with HubPages, with the publisher id ca-pub-xxxxxxxxxxxxxxxx. Are you sure you want to associate a different account? In most situations, Google only allows each person to have a single Google AdSense Account" Following the Google adsense terms and conditions you can change the associated adsense account by loging in with mail id of publisher ID to be replaced.