Thanks for your quick replay.
I want to upgrade MODX 2.3.1 to 2.5.5.
Is there any major changes? Will it happen any functions that are working on MODX 2.3.1 and not working on 2.5.5
How can i upgrade my current version to latest one?
Install the UpgradeMODX extra in Package Manager and use it to do the upgrades.
Important: Upgrade to 2.4.0, then 2.5.0, then 2.5.5.
It depends on the functions. Some extras may need to be upgraded. A few very old extras that have not been maintained could fail, but it's pretty rare.
Back up and test as you go. Especially - Back up your current site (files and database).