How to edit vertices in object's space?

Hello, I’m learning want to create simple terrain editor using OpenGL ImGUI. I know how to generate a flat plane that consists of triangles but I can’t find any resources that tell me how to edit it’s vertices in (I guess) objects space later. For example when I want to create mountains or course of the river. It would be done using some kind of patter loaded from a file (sth like a height map) How to do it?

its you job to figure that out … there are many ways to to that.
what you need is some kind of “picking” vertices … picking can be done in several ways
another thing is the tool to “manipulate” vertex data … this too can be done in several ways