I have two df's

df1:

```

index    text
1        string1
2        string2
3        string3
4        a
5        string5
6        a

```

df2:

```

index    text
4        string4
6        string6

```

How can I replace the text 'a' in df1 with the values found in df2 based on the index, so that it shows as below.

df3:

```

index    text
1        string1
2        string2
3        string3
4        string4
5        string5
6        string6

```