How do I accept Bank Transfer or UPI and confirm the customer has paid?

Both are offline methods in the Payment Methods table on Settings → Store Settings → Payment Gateway. Bank Transfer needs the Bank Transfer plugin. UPI is always there.

1. Click Configure. For Bank Transfer, enter the Account holder name and Account number (other bank details are optional). For UPI, enter your UPI ID; the QR code is generated for you. Tick Enable at checkout.
2. Under Payment proof (Bank Transfer), choose What the customer submits (reference, screenshot, both, either, or none), Where they can submit and the Order status while unverified ("Pending" or "Awaiting verification").
3. After paying, the customer submits proof on the confirmation page or in My Orders.
4. Open the order. The Payment proof panel shows the reference and screenshot. Click Approve (mark paid) to move the order to Processing, or Reject, giving a reason the customer will see.

Rejecting a proof cancels the order. WCart doesn't notify you yet when a proof arrives, so check these orders yourself.