Hi @mark2741
First, just wanted to let you know that there is an ACI specific related questions subsection - https://community.cisco.com/t5/application-centric/bd-p/12206936-discussions-aci
Regarding your question, you can change the uplink ports (40/100G ports) to downlink on the leaf, from:
Fabric -> Inventory -> Pod X -> Leaf X (click on the leaf name) -> Interface tab -> Change the mode to "Configuration" in the right corner, select the desired port(s), then click on "Fabric" to change it's operational usage.

Stay safe,
Sergiu