While deploying a class with its test class( coverage 100%) I got error msges. The errors are in the test classes which have already been deployed into the production. It said the code coverage for 4 test classes which are already in production is less than 75%. I made the necessary changes and made the code coverage more than 75% for all the 4 test classes and tried to deploy the classes. But again am getting the same error messages.

How to solve this problem?

Please help me.