Yes you should replace, 100K miles is the first replacement interval

Coolant is measured when cold, so normally the coolant level should be between Low and Full lines. Yes, sometimes the fluid level rises when hot (and some cars actually provide separate lines for hot and cold readings), but in this case Lexus says measure when cold.

There is a drain valve at the bottom of the rad, you can simply drain the coolant from the rad and then fill up with new coolant. If your reservoir is pretty much empty, just fill it up too. Usually suction is used for these as they are a bit harder to drain, but since you're low already might not even have to bother, just fill.

Youtube probably has a dozen how-to videos for every model of car out there, you can probably find something helpful to show you the steps.

 

Sorry wrong forum