If you are currently in M.6, you can apply for 2016 academic year. Every document has different content inside. It's recommended to go through all of them in order to understand APU curriculum and application process.
Overview tells you about APU in broad picture and curriculum provided.
Application guideline explains about how to apply to APU
And finally application form can be downloaded and printed. You can fill in this form and submit to Thai office.
Shall you have any further questions, please feel free to contact our Thailand office : )