You could use list_1.index('a') to get 'a' index if there was only one instance of that letter. But as I might see you have duplicate values in your list so for loop should work for that.
list_1 = ['a', 'b', 'c', 'a', 'b', 'c' ]
list_2 = ['1', '2', '3', '1', '2', '3']
indexes = []
search_value = 'a'
for e, value in enumerate(list_1): # e is basically our counter here so we use it later to find current position index
if value == search_value:
indexes.append(e)
if len(indexes) > 0: # check if our indexes list is not empty
for index in indexes:
list_2[index] = search_value
print(list_2)
Which will result in :
['a', '2', '3', 'a', '2', '3']
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