There is a small difference between the phrases “to be down for" something and “to be up for" something, though we often use them the same way in casual, spoken English. 
 * "Up for" something: This phrase generally indicates that you’re willing or ready to do something. It's often used when someone is considering whether to engage in an activity or is open to the idea. Example: “I was sick yesterday, but I’m feeling better today, so now I’m up for going to the movies.”
 * "Down for" something: This phrase is more casual and means you are not just willing to do something, but you are also excited or very interested in doing it. 

 Example: "I’m totally down for a road trip this weekend!"

 In everyday conversation, the difference is not very big, and you can use both phrases in a similar way. But “down for” might sound a bit more casual or excited.