I'm trying to download or upgrade the plugins in JMeter Plugin manager window, the following error is displayed.
Failed to apply changes: Cannot apply changes: Have no write access for JMeter directories, not possible to use plugins manager. C:\Program Files\apache-jmeter-5.2.1\apache-jmeter-5.2.1\lib\txt
Can someone tell me how to give write access to the respective file/folder?
It seems that your current user doesn't have write access to Program Files directory, in Windows only members of Administrators group can write to that folder (or you need to modify permissions for your user and provide write access to C:\Program Files\apache-jmeter-5.2.1\apache-jmeter-5.2.1 folder
It should be sufficient to run command-prompt as the administrator and launch JMeter from that command prompt instance so it would have elevated privileges and have write access to "lib/ext", "lib" and "bin" folders of JMeter installation.
Also it is not required to have JMeter under "Program Files", you can install it to your home folder where you should have the full control.
More information: How to Install the JMeter Plugins Manager