I haven't done it myself (but I need to do it at some point to check out my non-working headlamp washers !) but I don't think it's that hard. It's slightly easier if you have two people just so you've got an extra pair of hands, and to help avoid scratching the bumper as you take it off - make sure you put it down on a surface that's going to be kind to it.
No, you certainly don't need to take it to Lexus for that.
Having said all that, in the link I posted up about the temperature sensor, it doesn't actually say you need to take the bumper off, so if you jack the front of the car up (or put it on ramps), it may well be that you can get to the sensor from there.