abc.loc[3]=[6,19.4]
This command takes in the index of the row, where the values are to be replaced. Since the index of 4th row is 3, we'll assign abc.loc[3]=[6,19.4]
This gives us the desired result:
print(abc)
cyl mpg
0 4 21.0
1 6 21.0
2 4 22.8
3 6 19.5
4 8 18.7
5 4 18.1