in a few very easy to follow steps you will be able to undelegate your steem power.

1. Create a custom URL using steemconnect.com. It should look like this

https://v2.steemconnect.com/sign/delegateVestingShares?delegator=youraccount&delegatee=otheraccount&vesting_shares=0.000000%20VESTS

Where the URL shows "youraccount" remove that and enter your account name. where it shows "otheraccount" remove and enter the account name you have delegated SP to.

"0.000000%" at this portion of the URL use a "Sp to vest calculator", calculate the amount you want undelegate, remove the Zeroes only and add your number.

Now that you have your URl, open a browser, enter the link and hit enter.
You will be in Steemconnects "sign in" page. Enter your username and "key" and hit enter. you have successfully undelegated your sp. But one thing to remeber that it will take atleast seven days for the sp to return into you account.

@raised2b has a wonderful step by step tutorial about how to undelegate your sp. if you find difficulties in this answer feel free to go check out his post.