When an Execution Run is completed in SilkCentral Test Manager; the user can modify the Build Number associated with the Execution via the Activities view by clicking on the "Build Number", as shown in the screenshot below:

However; when the Build number is modified in this location; the change is not just applied to the current Execution, but for all older Executions against the same Build number:

 We can also see that the original Build for the Execution can still be seen in the 'Details' for the Execution Run:

This behaviour is the expected behaviour when modifying the Build Number through the 'Activities' view. Specifying another Build modifies the current Build value assigned to the Execution Definition, as opposed to creating a new one. For this reason the change is propogated to all Execution Definitions that have the original Build assigned.
 
The recommended process is to create a new Build in the following location "Administration | Products, Versions and Builds" and then assign it to the Execution Definition via its Properties, prior to running the Execution Definition: