To access a custom column set, change you layout to Table or Map + Table (if required)
To delete a custom column set:
1. Select the Columns Button and select the column set you would like to delete.
2. Once the custom column set has loaded in the search window, click the Columns heading again and select Customize. This will open the Column Details window.
3. In the Column Details window, choose the settings icon at the top right-hand corner.
4. In the drop down menu, click Delete.
TIP: You can also use this settings drop down to create a copy by selecting Save As, Share (with a Team), or create a New column set.