Hi my baby is 6 months old. She is rolling to her tummy and not back. In her sleep she is rolling onto her face and sleeping like that. Sometimes she turns her head but twice now she has been resting on her face with her nose flat. Any advice?

Most babies will be rolling over by 4- 5 months. This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.

Ensure that the cot mattress is firm & flat & there is nothing soft or loose in his cot, especially as she is face planting at times.
You can try to turn her face if you see her with face down, but often, like rolling to tummy, babies will just go back to their position of comfort.
Always continue to place baby on back to sleep.
Increasing tummy time when awake may also helpful.
I assume her arms are free in a sleep bag, & is no longer swaddled/wrapped -  she needs her arms free.

Red Nose has the following information now that your baby is she starting to roll.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over