Hi Louie,
If this is a custom table that does not contain links to any other tables or is not linked to by any other tables, you could simply create a button to remove all data from the table. A Delete Record From Table action where RecordID Greater Than 0 would suffice.
If this is the standard Customer table or if the table is linked by / linked to other tables, you always want to avoid deleting data. Deleting these customers will not only remove them from Method & QuickBooks, it will break any links that exist in other tables to these customers. This includes transactions, activities, work orders etc.
Your best option is to import the list updating records that already exist in Method. The safest way to perform this would be to export your customers from Method including the RecordID field in your export. After updating the exported list, you can re-import it using the update existing records by matching key fields option on step 2 of your import. You will use the RecordID as the key field since this is a unique value.
If you already have an updated list that does not include the RecordID, you can select another unique field as the key field. Possibly the Name field. You will need to ensure that this key field matches exactly what is already in Method or you will end up with duplicate records.
I hope this helps.
-Audisho