You can edit an Activity by clicking the link containing the subject or type of the Activity. Once you click on the Activity link, the Activity dialog box displays so you can make the necessary updates. Any updates are automatically saved.
The Activity link can be found in several places. Click HERE to learn how to access the Activity links.
Once you have the Activity dialog box open, just simply click on the Account Name and change or add the new Account name.
Once you do this, the Activity will now be associated with the new Account Name.