There is no entry in the tree structure on the left side of Project Setup where you can find a list of all the Selection Lists. Therefore you need to find a property in a class that shows List as a Property Type. In the following picture you see for example that Pipe Line Group has been selected.
If you click on “Edit…” the Selection List Property dialogbox opens up and lets you make changes to all the Selection Lists.
Before closing the dialogbox make sure that you have selected the Selection List of the property when you clicked on “Edit…”. Otherwise the wrong Selection List will be assigned to your property.
Also check the Default Value of the property, because the first value of the Selection List will be used as the Default Value and you probably want to have a blank Default Value.