Welcome to the ODK forum, @abdulrazzaktamim! We're glad you're here. When you get a chance, please introduce yourself on this other forum thread. I'd also encourage you to add a real picture as your avatar because it helps build community!
Have you taken a look at the documentation? There's a section on Installing Aggregate and also a section on Upgrading Aggregate. Do you have specific questions or issues not covered in the documentation?
If the server is offline, uploading submissions from the phones will fail. But you can retry as many times as you want until the server is online and the upload succeeds.