alist = [[1,2,3], [4,5,6], [7,8,9]]
blist = [['a','b','c'], ['d','e','f'], ['g','h','i']]
item = 1
z = [(i, x.index(item)) for i, x in enumerate(alist) if item in x][0]
alist[z[0]][z[1]] = blist[z[0]][z[1]]
print alist
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