Bring payments from the processor you used before NMI.
Go to Settings → Apps & data → Import payment history and choose where you are importing from.
- Stripe: create a restricted, read-only key and paste it. Payments, customers, refunds and disputes are imported.
- Square: create a free application at developer.squareup.com and paste its production access token.
- PayPal, Authorize.net, Braintree, Clover, Shopify Payments, QuickBooks Payments, Helcim, Stax and others: download a CSV of your transactions from them and choose the file. Columns are matched automatically; check them before importing.
Imported payments are labelled with where they came from and count in reports and customer history. They are records only: refunds for them happen in the old processor. Importing the same source again replaces it, and each source can be removed on its own.