From any community, choose the Activity Library in the side nav menu.
In the Units menu, choose any premium unit you wish to add to your library. To view the full unit, click on the View link in the unit card.
You can add it to your library by clicking the three dots on the corner of the unit. Then click on 'Create a copy' (Note: You can also add a unit to your library in view mode by clicking the three dots on the upper right top right corner.)
You'll be asked to choose a community where you want to save the copy. After choosing a community, click on 'Copy Learning Activity' at which point a pop-up will appear where you can click on "Yes, copy!"
When you switch over to your My Library option, you will see the new premium unit added to your library.