How can I fix the flat spot on my 6 month old baby's head?

Depends. This issue has emerged since the 90's when we shifted to back sleeping to avoid SIDS. Many self correct when they begin to sit upright & get out of the position that prolongs it.Some who sleep leaning to one or other side develop a neck muscle issue and need stretching exercises.Rare kids might benefit from a helmet.These treatments are all dependent on a good evaluation done with your doc.
Craniosacral therapy. Cst or craniosacral therapy works wonders for those flat scalps. Look at upledger institute website for a practitioner in your area.