After upgrading magento from 1.9.0 to 1.9.1 the ccavenue payment module is giving error
Fatal error: Call to a member function isLockExists() on a non-object in /public_html/app/code/core/Mage/Index/Model/Lock.php on line 257
How to fix this should I install new version of ccavenue module ?
Not sure why you think this is related to ccavenue payment module as the error does not appear to be relating to that at all. A simple google yields that you should clear all your caches and then re-index all your data which will hopefully correct your problem. Side effect of 1.9.1 upgrade by the looks of things.
To clear the cache go into your magento directory. Navigate to var/cache/ and delete all the contents of the cache folder.
Then to re-index everything you need to login to the admin and navigate to System-->Index Management. Select all the indexes and then re-index them all. I would clear the cache again afterwards and then try and load a page.