how to replace the front transmission seal?

First, the transmission will need to be removed. Then remove the torque converter, this is where the seal will be, also I would look at the front pump seal O ring as well because it can leak from there as well and look like the front seal is leaking. When putting the transmission back in be sure to get the torque converter installed correctly. This video will help:

https://www.youtube.com/watch?v=ZC7xHVMKLRw

The job takes special tools FYI. Check out the images (below). Please upload pictures or videos in your response of any problems so we can see what to help you with.