updated_txn_list = list(map(lambda x: x, filter(lambda y: y["paymentMthd"] == "Cryptocurrency", txn_list)))

print(updated_txn_list)

# Output
# [{'paymentMthd': 'Cryptocurrency', 'totalAmt': 3339.85},
# {'paymentMthd': 'Cryptocurrency', 'totalAmt': 576.15},
# ...]