It's pretty simple you go to

https://steembottracker.com/delegation.html

And write down the name of the account from which you want to stop the Delegation. Hit Load and it will show the active Delegations from that account.

Now go to the one you want to stop the Delegation from and hit remove. A pop up will open immediately and you will be asked to verify the transaction on Steem connect with your credentials.

Now once you undelegate it will take 5 days for you to get back your SteemPower. It's kind of a cool down period.