and a warm welcome to our forums.
It's an interesting question and you were very wise to ask it, before just plugging in a different AC adapter/charger.
If the "new" 65 w version turns out to be insufficient to power up your laptop and recharge your battery at the same time, then it may just fail to recharge the battery in a timely way..... but otherwise will do no harm, if the current is greater say 90w then this will be Ok as it's your computer that will draw (or try to) the correct current for its needs.
So a more important feature is the voltage that the different AC adapter/charger provides, to be sure of the exact voltage required for your particular make and model check in the owner's manual as there are different models of HP's popular U HP15's available. It is also often incribed into the plastic of the case ( but you may need a hand lens to read it unless you have 20/20 vision!) and the "old" AC adapter/charger will have the output voltage on its label, but here again you will need good eysight or a hand lens to read it!
Most of these models require 19.5 Volts to run satisfactorily (do not exceed that voltage) but a unit that produces up to a half a volt lower( say 19V) will often provide satisfactory service.
As to the risk of things "blowing up" its the excess voltage that does the damage (not the current from your replacement AC adapter/charger so keep to a maximum voltage specified in the handbook or on your old unit or of 19.5 V and you should be OK)
Post back if you need any extra information.