I added a person who is already on my tree under a different name

Adam Herbst_1

How do I combine two people or delete one instance of them.

dontiknowyou

You are asking how to merge duplicate profiles. The Help Center has lots of support pages about merging:

https://www.familysearch.org/en/help/helpcenter/family-tree

Sanra

It would be helpful to know the names, and ID numbers of the individuals. If you merge the two records, as suggest above, one of them will be deleted.