We have already built an application in AEM 6.2 (SP1) now we get client requirement upgrade it to the AEM 6.3.
Can you please share the list of task and analysis which we will do in the upgrade process.
You need to do preupgrade tasks followed by migration and later an inplace upgrade cycle to complete the process
see (1) for complete process
(1) - Upgrading to AEM 6.3