1. Create a custom field
2. Edit the following SQL query to you're preferences
Update tblAssetCustom
Set Custom1 = 'Location X' -- Change custom1 to the custom field you want to edit.
Where AssetName Like '%Assetname%' -- Change %AssetName% to you're preferred Assetname
As you can see, it isnt hard to change the custom fields in bulk. You just need a little bit of SQL knowledge and youre good to go.