Adding components to an already uploaded change set
Key Takeaway
An uploaded change set is locked. To add a component you forgot, clone the change set, add the component to the clone, and upload the clone.
A change set moves customizations (metadata) from one Salesforce org to another. Uploading a change set locks it, so you can't fix it in place when you notice a missing component. Instead, you send the missing component in a clone of the change set.
Outbound change sets
You build an outbound change set in the org you are logged in to. You then send it to another org. Its typical use is to send customizations that were created and tested in a sandbox to a production org.
Uploading a change set does not apply the changes. The target org must deploy the change set before anything takes effect.