How do you change front transmission seal?

You must remove the transmission and then remove the seal. Most times when a front seal is leaking it is because the bushing that supports the torque converter is worn, so it is wise to replace that as well.

My 2 cents - are you certain it is a front seal? They are not particularly common for failure on an A4LD transmission. Certainly anything can happen....but I would recommend before anything else to really verify the source of the leak first. If it is not obviously from the bellhousing area, look further.