updated_txn_list = list(map(lambda x: {
                      'paymentMthd': f"{x['paymentMthd']}_2024",
                      "totalAmt": x["totalAmt"]
                   }, txn_list))

print(updated_txn_list)

# Output
# [{'paymentMthd': 'Cryptocurrency_2024', 'totalAmt': 3339.85},
# {'paymentMthd': 'Cash on delivery_2024', 'totalAmt': 872.52},
# ...]