There are two ways to change a build: keep talking in the thread you are in, or reopen a saved build later and carry on.
In the same thread
Send a follow-up under the build card. The Lab sees the current script and returns a new card with the change applied. Press Stop while it is writing if you want to interrupt a reply.
From a saved build
In Your indicators or My builds, open a build with Iterate. It opens in the workspace with the build loaded as the starting point, so your next message edits it instead of starting from nothing, and the chart sits beside the thread.
Versions
Saving a change to a build you opened with Iterate adds a new version to the same build, shown as v1, v2 and so on beside its name. QuantLab keeps your 50 most recent versions and drops older ones automatically, so download any version you want to keep forever.