Obstetrician Leonie says:

An ideal position for a baby at this stage is head down, with his spine upwards, towards your tummy. However, your baby is lying head down, with his spine towards your back. But don't worry, this is quite common, and most babies turn before labour starts.
Sitting up straight and spending time in an all-fours position each day can help your baby to turn.
If this doesn't work, and your baby is back-to-back when labour starts, it's important to stay upright and mobile to help the baby turn.
Most back-to-back babies will still turn, and you will be able to deliver normally. But if the baby doesn't turn, delivery by C-section will usually be required.