I have a CDT and need to update its values to DB frequently. How do I update onc

Check out our documentation on the write to datastore smart service. Specifically, look at updating data in data store.

forum.appian.com/.../Write_to_Data_Store_Entity_Smart_Service

As long as the pv of type CDT has the value for @Id provided, the smart service will complete an update instead of insert.

In our case we have a combination of multiple columns that make up the unique key. How do we deal with that?

Composite keys are not supported in CDTs: forum.appian.com/.../Database_Schema_Best_Practices
You would have to use Query RDMBS smart node to operate on tables with composite keys.

This may not be an elegant solution but you can create another column in your table that stores the composite key as a single column value. You can use this to Map to the cdt. The challenge here is that it pushes the onus of creating and managing the pk to the developer instead of database. Apart from that, the general recommended solution is to maintain a single p.k