For me, I would do no more than maybe reduce the height / weight, and maybe HP since a dwarf is still classified as a "medium class creature" for size. But if you are intent on a size change, simply change the size category from large to medium under race for unicorn (in the editor) and save as Unicorn,small to differ from the original.
IF you really want to further define, you can (as noted above) reduce the HP by a point a dice and the Min/Max height & Weight categories while there as well.
https://www.dropbox.com/s/6ijfxdf7g3...Small.PNG?dl=0
The above screen shot is for 3.5e but the application is basically the same..
Go to the tools tab as shown here. Select Show Editor from the pull down.
That will take you to the 5e version of the Editor.
Select Race tab and scroll for Unicorn and select. This will load the basic 5e version of a unicorn racial template. Note the file has yet to be named (as pointed to in graphic)
Rename Unicorn to something like Unicorn,Small and assign a Unique Id
Then Save AS under file ( I named the example 5eAnimals) this will create a User file that you can store all of your user created animals for future use.
Note Saving names the user file for you and assigns the Unique ID to the "new unicorn"
After you have made whatever corrections to the file you desire, be sure to check user sources under sources button at bottom right and hit the test now button for Herolab to load the file.
DLG