You need an adapter such as this one (or this alternate one, or this one) that has a 3/8-16 hole on one side and a 1/4-20 screw on the other.

You can get them without the large flat circle, but they are more for adapting light stands and such. For a head on a tripod the larger round base will provide more stability, which is the whole point of a tripod.

From the comments below:

Actually, it looks like maybe all you need to do is remove the adapter that's already in the head... see that slot for turning it? Actually, on second look, the insert seems a little big for that, but... check it out.

and

All we can see of the brass insert is the head -- there's no way to tell what size socket it fits into. I agree with @junkyardsparkle that the slots cut into the insert are clearly meant to let you unscrew it. I'd try a large flathead screwdriver rather than a coin.

and

always possible that the brass insert has the equivalent of a bolt head to stop you screwing it too far in... Looks like it's clearly intended to be removable, so I'd try unscrewing it before splashing out on an adapter that may not be needed.

Based on Tom W's review under the review tab of the Velbon PH-157Q at Adorama, if the 1/4-20 insert is removed another insert with 3/8-16 threads must be used:

works as advertised; had bit of a tussle trying to get 3/8-16 bushing shipped, as advertised, but Customer service came thru- finally ... suggest you confirm available bushing sizes B4 ordering this item ...