Please actually ask a question or make a request instead of posting disconnected phrases.
Here is an example of a question:
How can I change "She is writing a letter" from active to passive voice? Thank you.
Here is an example of a request:
Please tell me how to change "She is writing a letter" from active to passive voice.
Your example is a sentence that simply does not work in passive voice. It would be "A letter is being written by her," but no native speaker would ever say that in normal conversation or writing.