Performing a purge and replace with BookTrakker on Biblio is simple. The only important two things to remember is to change the filename, as shown in the image below, to something that includes the word "purge" ("Purge_1.txt", as shown in this example is perfect) and to check the "All Online Inventory" box when sending.

Make sure to choose All Online Inventory when sending, or your purge will not take effect.