bulk custom field updates for the list

Go to contact manager
Click Advance Search button
On the left side, find contacts belonging to lists (option is called List Membership)
Select a list
Click Search button on bottom
Select contacts you want to update (or select all of them by clicking on square at the header)
If you are changing multiple contacts, click on gear icon on top right. Note: You MUST select top gear icon, and not a user icon
Click on Edit Contact(s)