Not sure what you mean by as3 mesh… But if you mean some mesh data in class form exported from Prefab or one some geometry generated via code, it doesn’t matter. That you load an obj, md2, 3ds or generate geometry via code, once a mesh exists in away3D, there is no difference. They are all the same.
You can then access the mesh.geometry.subGeometries[indexofyourchoice].vertexData and alter/change update at will.
Note that if you run on 4.1 and higher, there is a difference between CompactSubGeometry and SubGeometry. Where the subgeometries of the geometry is either SubGeometry for <4.1 or SubgeometryBase for >4.1.
The “classic” subGeometry holds multiple buffers, so accessing vertexData getter returns ONLY the vertices, while the same method with a CompactGeometry case will return a single buffer with the entire mesh data (uvs, vertices, normals etc). This is done this way to reduce uploads for better performance. You can if you want to access only the vertices use the stripBuffer method.
* - stripBuffer(0, 3), return only the vertices
* - stripBuffer(3, 3): return only the normals
* - stripBuffer(6, 3): return only the tangents
* - stripBuffer(9, 2): return only the uv’s
* - stripBuffer(11, 2): return only the secondary uv’s
To update, you will have to either use fromVectors method, or better, if the updates are frequent, loop over the entire data buffer using offsets and strides.
In SubGeometry case, you simply alter the vertices and call the updateVertexData method with the updated buffer.