How to drop 0.2 V dc?
You can get LDOs (Low DropOut regulators) that have 100mV or even less required headroom. But, you'd have to be exquisitely careful not to drop another few 10s of mV in connectors or wiring and blow that headroom budget.

A buck-boost converter will convert up or down to give you the voltage you want.

It would probably be best to take a step back and realise that 'exactly 5v' is not possible, even in theory. There is always a tolerance. Does your 5v load really require a tighter tolerance than 5.2v? Most loads will specify +/- 10% tolerance, +/- 5% if they're really picky, which is +/- 0.25v. Before you design a way to drop your 5.17v, you need to make sure you can provide the accuracy your load demands. From your question, it appears you're not too picky about the difference between 170mV and 200mV.

Another direction is to look at your supply. Is it adjustable? Can you use a supply with the correct output voltage. Can you use a supply with slightly higher output voltage that will allow you to regulate down with more headroom?