TypeError: tuple indices must be integers or slices, not str

The code:

def update_transaction(transactions):
    x = input("Pick a transaction by index:")
    a = float(input("Choose a new ammount:"))
    b = input("Choose a new day:")
    c = input("Choose a new type:")
    i = 0
    transactions[int(i)] = (a, 'b', 'c')