On the DD-WRT version supplied by Buffalo, it's pretty straightforward, actually. Download the latest firmware image for the Friendly Firmware at
www.buffalotech.com/support-and-downloads/download, and extract the .sip file.+ Once that's done, log in to your router using its IP address. Once that's done click on the Administration tab in the top right hand section of the DD-WRT menu. When you get there, below that, you'll see 'Firmware Update" in the little sub-menu below. Click browse, and navigate to the firmware file (it will have a .enc extension, or will say ENC or FILE next to the name). Once you do that, click update, and go grab a cup of coffee.
I may not work for Buffalo, but I've been a technician specialising in routers and Linux administration for just under ten years. I don't speak for Buffalo, or any of their employees, nor are the ideas, thoughts, etc related in any of my posts to you in any way a reflection of Buffalo Tech.