Chem3D set bond length

It is written in manual that after selecting bond I can change it's length in "Structures -> Measure -> Set bond length", but I could see only "display bond length" option. What am I doing wrong?

The same question about bond angle - how could I change it?

In fact I want to rotate one cycle (in naphthalene) about C-C bound between cycles and calculate bond energies in new configuration. But I did not find tool to make it.. So I need to move atoms manually but bond lengths are not fixed (while I do not want to change them, only angles).

Thanks in advance for any help.