to sing over an instrumental which has very high notes and I have a baritone voice, how do I do it?
Why would anybody want to sing in an uncomfortable range for themselves?
The obvious answer is to change the key to suit, or not sing at all.
It will depend, to a degree, on what your range is. Most folk will have a range of at least an octave, and most songs will have the same. It's just a matter of matching up the two. I got fed up with folk saying 'I sing in C/E♭/G'. That's just not feasible. We should all be singing in the most comfortable key, and if the range of that key is too great for our range, then that song isn't for us!
There are (have been) occasions when singing in 4/5 part harmony, someone's part is too high or too low. That's the time when there has to be some experimentation - trying out different keys, to find a common one for all. Or someone changing their harmony to suit their voice as well as their last harmony, but allowing someone else to take over their part.
If you must sing in a particular key - there are reasons - most of them unconvincing, then your real option is to drop an octave, and if the quality doesn't suffer, use that register.