If I use the older version of Rita, will it help to appear for pmbok V6 based exam?
It will. Except for ITTO. Just do not follow the inputs, tools and techniques and outputs in Rita. For that refer to PMBOK. Do the chapter end questions of Rita. That will be useful.
How important is to remember ITTO?
Remembering the ITTO (inputs, tools &techniques and outputs) for 49 processes is difficult and at the same time you may get some questions around it. Gamble little bit. Do not prepare focusing on mugging up ITTO. That is not a wise decision.
How important is the knowledge of agile while appearing for PMBOK V6 based PMP exam?
An overall understanding will help. The agile based questions will not be considered for evaluation as of now.