Upgrading from a custom firmware like 3.40 OE-A to official firmware 6.35 on a PSP 1000 (1xxx) model is a multi-step process that involves several components. Please note that this process can be risky, and if not done correctly, it can result in a bricked PSP. Make sure to follow these steps carefully and understand the risks involved. Also, ensure that you have a fully charged battery or the PSP plugged into a power source throughout the process.