Need help with Async

So amazon web services has all their plug in commands in Async. I however need those commands to finish so I can get information on whether a file on the server is more up to date then whats already downloaded. Async seems to always finish last in the unity code structure, how can I beat this? Can I have a script start executing after the script that calls the Async command finishes? I could probably make this work using Update() but I am trying to limit the number of processes going to help with battery life and performance. I think I'm correct that having Update() will increase battery usage, if not tell me please or if theres a good alternative please tell me that as well. I am so close to having this app functional but this is screwing me up...