1999 accord ex knocking really bad.Should I replace the spark plugs and put in a higher octan gas

These things must be checked /spark plug gap/ space between the center firing pin and the electrode base use a plug gap tool to check from any auto parts store , distributor cap , and rotor maybe worn there's to much space/air gap and worn spark plug wire contacts inside,clean out the throttle body with fuel injection cleaner and put a new air filter in //when doing these things if your plugs are really worn out and you can't regap them then do a complete tune up on the vehicle along with the throttle body cleaning these parts needed are //////spark plugs and plug wires///distributor cap and rotor////air filter///gas filter///when cleaning the throttle body if you see dark brown gunk behind the throttle plate use the whole can to clean it out and put 2 cans in the gas tank with a premium 93 octane fuel from EX. BP,MOBILE, SHELL, EXXON AMOCO,CITGO////THESE NAME BRAND FUELS HAVE DETERGENTS IN THEM TO KEEP YOUR SYSTEM CLEAN ////DO NOT USE LIBERTY ,DELTA,VALERO,WAVE ,USA, OR ANY OTHER NO NAME FUEL BECAUSE YOU ARE DOING A LOT OF DAMAGE INSIDE THE CYLINDERS AND CATALYTIC CONVERTORS THAT YOU CAN'T SEE //PUT THE 2 CANS IN BERRIMANS IS THE BEST BUT GUM OUT WILL WORK TO FILL UP WITH 93 OCTANE AND NOW GO FOR A RIDE FOR 1 HOUR AT 55MPH OR MORE DEPENDING ON SPEED LIMIT OK, THE HIGHER THE OCTANE LEVEL THE SLOWER THE FUEL BURNS BECAUSE IT IS MORE STABILE COMBINED WITH THE 2 CANS WILL PRODUCE A MUCH HOTTER FLAME IN THE EXHAUST TO BURN OFF THE GUNK INSIDE THE ENGINE AND CATALYTIC CONVERTORS PLEASE SEE NEXT PART IN COMMENTS

With that many miles there is no point in putting synthetic oil in the engine because it will probably be too thin and cause the seals to start leaking. Keep the oil changed regularly (every 3000 miles) and use a brand name 5W-30 grade oil like Mobil1 or castrol. As far as gas goes I would put a tank of high octane gas in about once a month. You can try using high octane all the time but check your mileage per tank on the regular grade and then after you've filled you car about 4-5 times with high grade so you can see if it really is making a difference. Also check your owners manual to see if it recommends higher grade gasoline.

If it sounds like a sort of knocking noise, it could be that you are using a gas with to low of a octane level. See what happens if u put higher grade gas in or go to auto parts store and try a 5 dollar bottle of octane booster on your next fill up. If that quiets your noise that could be your problem.

Octane ratings have nothing to do with fuel economy or power. The octane rating only indicates how much the fuel air mixture can be compressed before it ignites without the spark plug firing in a hot running engine. Some really high compression engines require higher octane fuels so they don't knock when running. If you don't have a sticker under the filler lid that says you have to use one of higher octane fuels, then don't use high-test.
It used to be that using a low octane fuel in an engine designed to use high octane fuels could damage the engine.
Nowadays, engines are all computer controlled and most high octane engines will re-tune themselves on the fly so that they don't knock if the wrong grade of fuel is used.

Wrong spark plugs?
Make sure you use iridium plugs by NGK or Denso.
NGK V-Power Plugs BKR5E-11

Carbon Buildup?
Get some Techron and run several tanks

Wrong Octane?
Your engine should use regular gas. But you might be getting some low grade gas. You can try using a higher octane fuel. Or try adding a octane booster.

Hello: The Knock Sensors purpose is to detect pre-ignition, when the computer receives a knock signal it re tarts the timing to prevent the pre-ignition. Any mechanical related knock will activate the knock sensor signal. If the knock sensor is bad it WILL NOT cause the engine to knock.......

Humm, give it some time. there are upper cylinder cleaners, but I worry about converter damage, and oxygen sensor damage. The lower the octane, the hotter and more masssive the burn, exactly backwards from what you would think, but higher octane is slower burn and av.gas is really, really slow.
Ford has had slow to start issues that traced back to drivers using too high octane....

Still, if it keeps pinging, you may need to run higher than spec octane.
I would have thought that your car would have knock sensor that would trim this issue...

It may not be bad gas, but it could be the wrong octane rating for that engine. Check your owner's manual and see what octane is recommended for that engine. If you're running 87 octane and it requires 91 or higher octane, then you need to be running the higher octane fuel. By running lower octane fuel in an engine that requires higher octane, your getting pre-ignition, the knocking sound you hear. What's happening is that as the piston moves up on the compression stroke, the gas/air mixture is compressing and as the pressure goes up, so does the temperature of that mixture. It's possible that temperature can get so hot that the gas/air mixture ignites from the high temperature and not from the spark plug. Higher octane fuel will help this since higher octane fuel is harder to ignite. I say try running some 92 or higher octane fuel in your car and see if you still hear the knocking.

If it is the same plug every time, then you have an oil leak past the valve guides on that bank. It needs new seals.

Other than that, preventing knock in warmer weather can be done by doing such things as running higher octane or getting the motor to run cooler.

If the knocking did not go away with the higher octane fuel then you may have other problems. A dirty fuel injector oor a misfiring spark plug can also cause knocking in a modern car. A faulty sensor may also be causing the wrong fuel / air mixture in the cylinder... check out the cheapest fixes first.