MSBNX does not support standard "undo/redo" behavior for individual changes to models.
Instead, you may create temporary "snapshot" versions of an entire model as you make changes to it. If you later decide to "undo" the change, you can revert to a previous version by restoring an earlier "snapshot".
Conceptually, each model opened in MSBNX has a push down (LIFO) stack of saved model versions; each element of the stack actually corresponds to a temporary file on disk. When you "push" a model onto the stack, MSBNX makes a copy of the model in its current state into a file in a temporary directory. You may push as many times as you wish.
Later, you can "pop" a "pushed" version of the model. This has the effect of restoring it back to the exact state the model was in when it was "pushed".
The collection of temporary pushed model files is emptied every time MSBNX starts.
This function is available from the diagram window toolbar.