Login to Ad Manager. Scroll down to the bottom of the page and click on:

Click Zone Configuration on the next page:

Make sure you're in the right web property. If you're not, just click on the drop-down menu and select the property you wish to work on:

Go to the position and click the pencil icon to reveal the dimensions assigned to it:

To delete a dimension, click the trash can icon next to it:

Changes to dimensions are auto-saved.