How can I change the bond type (single, double) of an existing bond in Avogadro?
As discussed in the Avogadro website about the draw tool you can change the bond order:


By clicking on the bond - the bond order will go from single, to double, to triple, back to single.
By setting the bond order and "drawing over" the bond. The new setting (e.g., double) will replace the existing bond.