A lot of parameters that can be set by JVM arguments are available in the WebSphere Admin Console, but sometimes, you just want to set parameters with JVM arguments.

You have two options. First, you can use the admin console to set the parameters, and second, you could just update the server.xml file directly.

Changing the JVM through the admin console can be done by going to:
Servers > Server Types > WebSphere application servers > server1. Followed by Java and process management on the right side navigation menu and then Process definition Finally, under the Additional Properties menu, choose Java virtual machine.

The parameter and value can be added to the Generic JVM arguments section or as name-value pairs in the Custom properties page.
jvm parameters

The second option is to just edit the file directly:
The jvmEntries element has a genericJvmArguments attribute that can be modified or a new systemProperties can be added.