Have a 1999 lexus es300. Getting indications that knock sensors need to be replaced. Car has 144k miles. What do the knock sensors do and how easy / tough to replace?

If you have some common tools the knock sensors are no big deal.  If you take a lexus they will make is sound like you need to completely disassemble the engine and scare you off.
First, do you really have a problem with preignition causing real knocking due to carbon buildup, or diluted gasoline or do you have failed knock sensor(s).  Try to decarbon the engine first.
There is a technique used with water being sprayed into a warm engine intake.  This will clean up the valves and carbon buildup which can result in engine knocking.  

The job will be a couple hundred for the parts and a couple hours of your time.  After you are done, you have entitled yourself to approximately $500 in new tools for your tool chest since this is what Toyota and Lexus will charge to fix (about $1k+).

You will need to buy the following to address the knock sensors and a couple other items while you're in the neighborhood.
- 1x upper intake plenum gaskets - 2x lower intake plenum gasket - coolant bypass hose found in the same area as knock sensors - 2x knock sensors (bank1 and bank2) - short pig tail cable which connects both knock sensors to common wire harness. - toyota antifreeze  (2 gallons, if i remember correctly... when mixed to 50/50 (antifreeze/ water) you will have 4 gallons.
drain the radiator drain the front of the engine using the engine drain plug - this plug is on the front right side facing the engine, behind the right hand exhaust manifold. (this will lower antifreeze enough to avoid dumping antifreeze all over the engine later) remove the air filter box remove the connections into the throttle body remove the cable from the throttle control remove the upper air intake plenum remove the two lower air intake plenums remove the antifreeze fill port
Knock sensors will be in the valley between the lower intake plenums.  You will need to remove the rubber antifreeze bypass hose (little short hose blocking access to the knock sensors).
By the way, the reason you bought a replacement, is that if this short hose splits from normal wear, you need to spend this money and effort to reach it, so just do it now.
The little pig tail cable which connects both knock sensors to the wire harness is said to cause a big majority of the problem due to the plastic getting hot and brittle, falling off and shorting to the engine.  While in this mode the engine thinks knock is occuring and starts to retune the air fuel mixture to get rid of the knock until it goes into minimal operations mode.
After replacing the knock sensors (and tighten to torque specs), replacing pig tail cable, and reinstalling your new short by pass hose.  Replace the lower gaskets, antifreeze fill port, lower intakes, upper gasket, upper intake, air intake, air filter, reattach everything, reconnect all the rubber lines you pulled off, make sure there are no splits in the air intake passage anywhere, retighten the engine antifreeze drain plug, and the drain on the bottom of your radiator, and your done!
Don't forget the easy stuff.  Dilute the antifreeze with water and fill the radiator and overflow tub.  Run the engine for several minutes until its hot and opens the thermistat to allow antifreeze into the empty engine cavities.  Turn the engine off and get a cold one as the engine cools.  Once absolutely sure engine is cold, refill the radiator with antifreeze and refill the reserve tub.  Do this at until all the beer is gone and you are both full of antifreeze (in one form or another)..... obviously red stuff in the car, amber down the gut.