There are two easy ways to do what you want.
df = DataFrame(group=["foo", "notfoo", "foo"], data = 1:3)
Using replace
replace!(df.group, "foo" => "bar")
| group | data | |
|---|---|---|
| String | Int64 | |
| 1 | bar | 1 |
| 2 | notfoo | 2 |
| 3 | bar | 3 |
Or by indexing as you do in your example
df[df.group .== "foo","group"] .= "bar"
Both of these will do the change inplace.