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"https://v2.steemconnect.com/sign/delegateVestingShares?delegator=youraccount&delegatee=otheraccount&vesting_shares=0.000000%20VESTS
"
Remember that it will take 7 days for the Steem Power to be available for you to use again but if you check steemd you'll see that you have a new undelegate transaction.

from: https://steemit.com/steemit/@raised2b/how-to-undelegate-steem-power-in-4-easy-steps

I guess, you can either use Vessel wallet or change delegation with zero in it :D
or use this link https://v2.steemconnect.com/sign/undelegate-vesting-shares?delegatee=jerrybanfield

thanks!