Hello, I understand your concern This is called positional plagiocephaly It is usually harmless and does not affect brain development. In most cases, the shape improves on its own as baby starts sitting, crawling, and spending less time lying down

Consult with Pediatrician physically for further evaluation and treatment

It will correct with increasing age . As baby sits up , stands . Give tummy time for 2-3 times a day . Under supervision during day time .

A flat backside of head is more liable to positional correction before 4 months of age. Nevertheless, the skull bones are still soft and pliable to correction since the baby at 6 months has still not started to sit up and be off the bed much. Please keep putting the baby to sleep on the back till 1 year of age to prevent risk of SIDS. At the most you may turn the head towards both sides, as the case maybe, to prevent preferential side flattening of head. It is recommended to make use of window light/ a preferred toy/ mother’s presence to help the baby turn the head so. Tummy time is another good option. You may hold the baby occasionally in the arms to help sleep off without support on the back of head. With age, some correction will take place as the baby gets to sit up, stand and walk around

Greetings kindly go through the article you will get idea "Flat Head Syndrome in Infants." by Dr.Shebin C E - https://www.practo.com/healthfeed/flat-head-syndrome-in-infants-51654/post

Mild flatness in a 6-month-old often improves on its own as the baby grows, sits up more, and spends less time on their back. Since you’ve tried tummy time and position changes, continue them, and monitor progress. If there’s no improvement by 7–8 months, consult your pediatrician—helmet therapy may be considered if needed.

It is a slow and tedious process. Do not expect results in 10-15 days. Just like it took a long time to flatten it will also take a long time for it to become more rounded. Also, check if the father also has a flatter backside as it can be familial as well.