I don't see an option to change the credentials for an already added compute manager.
There's not one in the UI, you'd have to delete and re-create the compute manager.
Or you can use the API:
Updates a specified compute manager
Request:
PUT
/api/v1/fabric/compute-managers/<compute-manager-id>
n/a
n/a
ComputeManager+
Example Request:PUT https://<nsx-mgr>/api/v1/fabric/compute-managers/cf1f01db-e5b3-4688-9c1e-5f47d335fb01
{
"id": "cf1f01db-e5b3-4688-9c1e-5f47d335fb01",
"server": "10.112.3.45",
"origin_type": "vCenter",
"credential" : {
"credential_type" : "UsernamePasswordLoginCredential",
"username": "user2",
"password": "somepassword",
"thumbprint": "3A:21:22:A6:72:28:DA:EC:9D:05:AF:0B:B5:C6:44:CC:52:8D:54:AC"
},
"_revision": 0
}