<<We are running 32bit Firebird 1.5.6.5026-0 CS on RHEL OS.
May I know what our upgrade path should be to bring our firebird databases to the latest version?
How will the performance be affected?
What are the advantages?
Can someone please advise>>

This will help.

https://www.ibphoenix.com/files/conf2016/upgrading-Cisar-2016.pdf

Regards
Paul