Manipulate UTF8 Value of a bson_t document

Hey there.
I am using the Mongo C Driver and I would like to overwrite values in a bson_t document locally.

So far:
I receive a cursor with mongoc_collection_find_with_opts() and mongoc_cursor_next() places a bson_t pointer on the desired bson document.
There are some functions provided like bson_iter_overwrite_int32() and bson_iter_overwrite_double() with which I can manipulate the value of an entry with the datatypes int32 or double for example but I would like to overwrite a string.
Unfortunately I can not find a function for this. Is there a possibility to manipulate a value of the datatype string?