Question

she is singing a song change into passive voice

Answer 1

A song is being sung by her.

You need to understand the rule of the Active and passive voice as it is not just inter-changing of the sentence.

The object will become the subject and the subject will turn out to be an object or an agent in the passive voice.

Answer 2

She is singing a song.

The given sentence is in active form.

To convert the active sentence into passive form, the "object of the active" sentences is considered as the "subject of the passive" sentence.

Here is a given sentence is in present continuous tense. So, the given sentence is converted into "present perfect continuous" tense.  

"A song is being sung by her" is the passive form of the give active sentence.