How do you add/remove terrain trees during gameplay?

I need some help, how do I add/remove/edit terrain trees during runtime? Whenever I try accessing the TreeInstances and edit, nothing happens(I do not know why). Another thing is, is it expensive to add/remove terrain trees at runtime?
Thanks in advance!!!

See this

which is written to answer how do I ‘interact’ with trees but it shows the Tree structures involved; at the bottom in the Comments sections are additional links with examples of modifying tree data at runtime which is an expensive operation for most configs.