In order to update all the documents in a collection first you have have to retrieve all of them in a List and then iterate that list and update them. Here is a sample code:-
for (int k = 0; k < list.size(); k++) { firestore.collection("Events").document(list.get(k)) .update("Key", value) .addOnSuccessListener(new OnSuccessListener<Void>() { @Override public void onSuccess(Void aVoid) { Log.i("Update", "Value Updated"); } }) .addOnFailureListener(new OnFailureListener() { @Override public void onFailure(Exception e) { Toast.makeText(MainActivity.this, "Error In Updating Details: " + e.getMessage(), Toast.LENGTH_SHORT).show(); pd.dismiss(); } }); }

Just out of curiosity I've taken a look at what happens under the hood, and I've used [dtruss/strace][1] on each test.
C++
./a.out < in Saw 6512403 lines in 8 seconds. Crunch speed: 814050
syscalls `sudo dtruss -c ./a.out < in`
CALL COUNT __mac_syscall 1 <snip> open 6 pread 8 mprotect 17 mmap 22 stat64 30 read_nocancel 25958

Python
./a.py < in Read 6512402 lines in 1 seconds. LPS: 6512402
syscalls `sudo dtruss -c ./a.py < in`
CALL COUNT __mac_syscall 1 <snip> open 5 pread 8 mprotect 17 mmap 21 stat64 29
[1]: http://en.wikipedia.org/wiki/Strace