There are many tools to help you undelegate, and I really recommend @jesta's Vessel. Its my delegation tool of choice. It can be downloaded here: https://github.com/aaroncox/vessel/releases/tag/v0.2.0. Simply head over to the accounts tab once you have it downloaded, go to SP delegations and press the trash can to end the delegation.

You can also use https://steembottracker.com/delegation.html to view your current delegations and edit them. You just go to the site, type in your name and then press the delete button on the ones you want to remove. Once you do that, it'll open up a steemconnect page asking for you to sign the undelegation. Also a good way to do it too.

It will take 7 days after undelegating to get the SPs returned to you so be wary of that.