Is there any way to modify the output voltage of a laptop adapter from 18.5V to say 7V or 13.6V?
If it is 230V AC adapter then it works with 400V DC internally, so yes, it is dangerous to mess with it for somebody who doesn't have experience. It can blow in your hands (trust me, I have experienced several such explosions, luckily with happy end).
It is much more safe to build buck regulator, that you are mentioning.  

It could be even impossible to modify that adapter, even if you decided to. It depends on the construction, some are build specifically for some voltage, with specific parts.