Does anyone one whether is it possible (and if so how)
to change the target sigmas of the H-bonds restraining base pairs?
It is not possible to change target or sigma values for H-bonds or H-bond angles. The values are picked individually depending on atoms involved and Saenger class of a base pair. It could be instructive for us to know why do you want to change them? Do you have any particular scenario where the standard sigmas do not work as you are expecting?
Or is it the case that parallelity_sigma is doing what I'm looking for?
parallelity_sigma is a sigma for separate restraint (parallelity) which has no influence on H-bond restraints.