Many times, CRM users tend to complete the activities without setting the regarding field to appropriate record.
Once the activity is completed, then you have no choice other than to delete that activity and recreate with same information and then select the regarding field.
But there is another way of doing this. You can create a manual workflow in CRM for activites and the action would be to change the status of the activity to Open.
Once you published your workflow, you can open any completed activity, you will see a new button called "Run workflow". Click on that, it will show you a list of available workflows, select the workflow you just created and click OK.
Once the workflow is executed successfully, the status of the completed activity becomes active. Now you can change any value on the form, like set the regarding field and again close the activity.
Easy itn't it!! Happy googling......