Turning the brim of the hat is easy knitting once you have the provisional cast on and have joined to knit in the round. I knit for two inches, knit one purl round and knit for another two inches. The piece will naturally fold on the purl round.
Take out the provisional cast on. First find the knot you tied earlier. Untie it and pull gently so that the loop pulls out, keep tugging and the crochet chain will pull out one stitch at a time, leaving live knit stitches. Place the stitches on a spare needle. In the photo the red arrow points at the waste yarn, remove the knot and pull the loop and the stitches will pop loose (green arrow). Place the stitches on the spare needle.
When the waste yarn has been removed and stitches picked up, you have needles and live stitches on both ends of your knitting. The red arrow points to the needle holding the provisional cast stitches. Fold the knitting along the purl row (green arrow) and line up the needles.
Knit the brim together by knitting one stitch from the front needle with one stitch from the rear needle.
Red Arrow shows row after brim has been knit together.