If you're familiar with the process of delegation in the first place, you generally have to input a number of VESTS that you would like to delegate. If you're using SteemConnect to delegate, simply set the VESTS amount to zero.

Alternatively, you can check out the Vessel desktop wallet developed by @jesta. It makes delegating and revoking delegation very simple. There's a slider for VESTS instead of you having to perform the calculation yourself. You can get the latest version here:

https://github.com/aaroncox/vessel/releases