There are a couple of reasons why you may want to change your controller to a different account. E.g.:

1. You don't have any funds left in your controller to pay transaction fees
2. Your old controller account was compromised
    

Here is how to change your controller account: 

1. Navigate to Network --> Staking --> Account actions
2. Find your Stash account 
3. Click on the three dots next to it
4. Select "Change controller account"
   
   
   
   
5. In the "controller account" field, select the new account you want to set as controller.
   
   If you want to simply unlink your current controller from the stash, then select your stash account in this field. (In that case the stash and the controller will be the same account.)
   
   
   
6. Click on "Set controller" to finalize the transaction.
   
   

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