This is the rendering code to render and do a pick test on the terrain

What i want todo next is increase the picked triangles vertices for mouse 1 and decrease for mouse 2 , so that i can sculpt it to how i like

I know i have to lock the vertex buffer but i am unsure how to get the positions and adjust them and when todo it

// Clear the backbuffer and depth buffer.
    HR(gd3dDevice->Clear(0, 0, D3DCLEAR_TARGET | D3DCLEAR_ZBUFFER, 0xffeeeeee, 1.0f, 0));

    HR(gd3dDevice->BeginScene());

    // Setup the rendering FX
    HR(mFX->SetMatrix(mhViewProj, &(mView*mProj)));
    HR(mFX->SetTechnique(mhTech));
    UINT numPasses = 0;
    HR(mFX->Begin(&numPasses, 0));
    HR(mFX->BeginPass(0));
    HR(gd3dDevice->SetRenderState(D3DRS_FILLMODE, D3DFILL_WIREFRAME));
    HR(mTerrainMesh->DrawSubset(0));


    // Switch back to solid mode.
    HR(gd3dDevice->SetRenderState(D3DRS_FILLMODE, D3DFILL_SOLID));

    // Did we pick anything?
    D3DXVECTOR3 originW(0.0f, 0.0f, 0.0f);
    D3DXVECTOR3 dirW(0.0f, 0.0f, 0.0f);
    if( gDInput->mouseButtonDown(0) )
    {
        getWorldPickingRay(originW, dirW);

        BOOL hit = 0;
        DWORD faceIndex = -1;
        float u = 0.0f;
        float v = 0.0f;
        float dist = 0.0f;
        ID3DXBuffer* allhits = 0;
        DWORD numHits = 0;
        HR(D3DXIntersect(mTerrainMesh, &originW, &dirW, &hit,
            &faceIndex, &u, &v, &dist, &allhits, &numHits));
        ReleaseCOM(allhits);

        // We hit anything?
        if( hit )
        {
            // Yes, draw the picked triangle in solid mode.
            IDirect3DVertexBuffer9* vb = 0;
            IDirect3DIndexBuffer9* ib = 0;
            HR(mTerrainMesh->GetVertexBuffer(&vb));
            HR(mTerrainMesh->GetIndexBuffer(&ib));

            HR(gd3dDevice->SetIndices(ib));
            HR(gd3dDevice->SetVertexDeclaration(VertexPNT::Decl));
            HR(gd3dDevice->SetStreamSource(0, vb, 0, sizeof(VertexPNT)));

            // faceIndex identifies the picked triangle to draw.
            HR(gd3dDevice->DrawIndexedPrimitive(
                D3DPT_TRIANGLELIST, 0, 0, mTerrainMesh->GetNumVertices(), faceIndex*3, 1))
            
            ReleaseCOM(vb);
            ReleaseCOM(ib);
        }
    }
    HR(mFX->EndPass());
    HR(mFX->End());
    
    mGfxStats->display();

    HR(gd3dDevice->EndScene());

    // Present the backbuffer.
    HR(gd3dDevice->Present(0, 0, 0, 0));

I know i have to lock the vertex buffer but i am unsure how to get the positions and adjust them and when todo it

Given a face index (say FI), you first get the 3 vertex indices from the index buffer. E.g.,

UINT vidx[3];
vidx[0] = ib[ FI*3 + 0 ];
vidx[1] = ib[ FI*3  + 1 ];
vidx[2] = ib[ FI*3 + 2 ];

Then adjust the position value for each of those three vertices. E.g.,

for( int i = 0; i < 3; ++i )
{
   vb[ vidx[ i ] ].position += D3DXVECTOR3( ... some offset ... );
}

As Burnt_Fyr suggests, if you keep a copy of the vertex and index buffers, you can do those calcs on the copy, then lock the vertex buffer, update just the 3 vertices you've repositioned, unlock the vertex buffer and be on your way.

when i do this

UINT vidx[3];
vidx[0] = ib[ faceIndex*3 + 0 ];
vidx[1] = ib[ faceIndex*3 + 1 ];
vidx[2] = ib[ faceIndex*3 + 2 ];

You need to look at the intent of the code and try to understand it, rather than just copy-and-paste.

Lock the index buffer (if you're not using a copy), and cast the data pointer to a pointer to the index buffer data format. Use that pointer instead of ib in the code snippet. Be sure to unlock the index buffer when you're done with it.

Similar for vb. vb in the code snippet is intended to be a pointer to VertexPNT (if that's the format of your vertex structures).

where does .position come from?

You declare your vertex structure as VertexPNT. That normally means the vertex is comprised of a Position, a Normal, and Texture coordinates. Position is the position part of the vertex.