Welcome to the Forum.
I am unsure of what you mean by
It is not possible to ‘label’ part of a Design, each of the latter only supports one Name/Title/Label.
If you mean that you have created a Design, and have called it ‘#### Part 1’ and later decide that you have modified ‘part 1’ to be come ‘part 2’ within the same Design and need to give it a different Name/Title/Label then the following may apply.
Return to Designs screen showing your Designs as Thumbnail views.
With your Finger Tap and Hold until ‘part 1’ is Selected, then choose the option Duplicate.
Using the ‘copy Design’ remove all that is not applicabl;e to ‘part 2’ and rename the Design appropriately.
If you wish to have ‘part 1’ and ‘part 2’ reunited in the same Design, you will have to Export the appropriate Design so that you can reimport it.
If this does not help please let us know.