2003 Ford Explorer code P0775

When starting off I have to take my foot off gas so it will shift from first to second, the other gears shift fine except for the overdrive. The od/off flashes and I can not manually turn it back on! Check engine light came on and got a code of P0775, in which I read about it but I want to know the steps on how to fix it! I already changed transmission fluid and filter. Can u please give me step by step on what or how to fix and part numbers for any parts I need? And diagram would be a big help. I just got this vehicle and this all happened the two and day from when I purchased it! I tried looking up part numbers and how to fix but found nothing that tells me what Info I need! Thank you for your time!