Answer:

Observe that the following model has been created in AutoPIPE Vessel for a simple horizontal pressure vessel with ring stiffener (with Angle) for external pressure reinforcement:

To modify the stiffener, go to Execute>Convert Results as Input Data.

The stiffener can now be seen in the components list in the left:

Double click on Stiffeners and open the Stiffener dialog box and select another type of shape eg. I-shape and select the size from the tag, click OK:

Observe that now the stiffener type is changed to I-shape: