roopa We performed some customization in some pages.

Did you modify the core files of the product? Or do you mean that you have used the web ui to make changes?

If you did not modify the core files then you just upgrade like normally.

Here are some slightly dated instructions (but nothing has changed in the steps)
https://tmib.net/upgrading-osticket-1-10-x-to-current-1-11-0/

If you modified the core files and you want to try to keep them, then you will need to locate your changes.

Document them.
Then follow the normal upgrade proceedures.
Lastly you would attempt to re-apply the changes that you made.