1. Remove existing memory modules: Before adding new memory modules, the existing memory modules should be removed.
2. Add 16x 64GB DIMMs: The new DIMMs should be added, in this case is 16 DIMMs of 64GB each, which will expand the memory to the highest configuration available.
3. Shutdown the Database Server if running: The Database Server should be shutdown before adding new memory modules in order to prevent any possible data loss or corruption.
The steps required to expand an Exadata X9M-2 Database Server with the lowest memory configuration available to the highest memory configuration are as follows:
Shutdown the Database Server if running.
Remove existing memory modules.
Add 16x 64GB DIMMs.
For reference, the Oracle Exadata Database Machine X9M Implementation Essentials Official Text Book states that "the lowest memory configuration for the Exadata X9M-2 Database Servers is 384GB and the highest is 768GB [1]. The memory upgrade is done in increments of 64GB using 16x 64GB DIMMs".

According to Oracle's documentation1, to set a threshold for a metric on a database server, you need to use the create threshold command with the appropriate parameters. The set threshold command is used to modify an existing threshold2.
Therefore, the command that you should use to generate alerts for such an event is:
dbmcli -e "create threshold ds_temp comparison='>', critical=30" on each database server1