I'm not sure what you mean by "changing what to back up".

If you are talking about excluding specific files or directories from backup
for a client, the way to do that without going to the client is called a
"client option set".

You can put your include/exclude statements in a "client option set" on the
TSM server, and then assign the client option set to the appropriate
clients.

And you can do that from either the admin command line, or the standard Web
admin client.