It's really up to you.

If you have just installed a mitigation system, you can simply restart your monitor so that you can erase all your Radon data and measure the new levels from scratch. Otherwise, you can simply keep running the same unit in the same position: you will then see that your daily and weekly average values are consistently lower than your long-term reading...and with time they will bring your long term average down.

It depends on what your objective is: either way, you will see new values being consistently lower than the old ones, which proves that your mitigation system actually works.