There are several ways to change atom type:
1. Select atom type button on toolbar, then click atoms to change their type
2. Mouse-hover an atom, then press keyboard key to change its type: C, N, S, O, l (Cl), b (Br), F, I, A, X
3. Select text tool on toolbar, then click an atom
4. Mouse-hover an atom, then press Enter key to show text edit box