You can do lots of things.
Hire Theologian. Park your army in that province. Enact policy. Make a decision. Convert province. Change culture.
No way to do anything? Seriously?
Have you played anything other than a European country?
Hire a theologian - ignoring you might not have access to one, probably too expensive. You get *at most* a single level 1 advisor for the first 50-100 years as most non-European countries.
Park your army - of 8, who reduce unrest by... wait for it... a whole 2... in one province.
Make a decision - what decision? For example, Kongo starts with 2 possible decisions (one of which requires a theologian), and neither do anything to reduce RR. It doesn't gain any with tech either. If you aren't Muslim or Christian, this is what your starting decisions probably look like. (Been too long since I played a Hindu country, they might be in slightly better shape).
Convert province - .... and if you already did that? Most non-Christian religions don't have a hate on for other religions that makes them a major source of unrest either. (And conversion provokes more unrest in the short-term, which if you aren't Muslim or Christian, is actually the medium (Hindu) to long term, because you have no decisions with missionary strength, assuming you can convert the province at all.)
Change culture - requires conversion first. Takes forever. Solves 2 RR. Really not a solution to unrest problems.
Now, on the flip side, if you get unlucky with an heir (or lack thereof) just once, you could go from 100 legit to 20 legit instantly, in addition to the -1 stab hit. That's a 6-7 RR swing (depending on whether the stab hit sends you negative or not). Bad luck could instantly give you mass instability in provinces you had safely reduced autonomy in 5, 10, or 20 years before. 25 years of revolts every 4-5 years, with revolt stack sizes bigger than your FL, is not fun. All due to something you had absolutely no control over. Assuming you can even afford to keep your army at FL.