Annoying "Under-voltage detected!" on RPI

During usual work, "Under-voltage detected!" message along with flash icon on right-up corner is popped up unexpectedly or frequently.

This message comes from linux kernel, and is caused by improper Power Supply, so can't be removable.

Only solution is that 

Get a proper Power Supply Unit (not just any phone charger nor your PC/Mac's USB port!)

If RPi3B or RPi3B+ : use 5v 2.5A and up, cable must be 18AWG or at least 1.0mmsq
if RPi2B : use 5v 2.0A and up, cable must be 18AWG or at least 1.0mmsq
if RPi1B+, RPiZ/W : use 5v 1.5A and up, cable must be 20AWG or at least 0.75mmsq
if RPiA+ : use 5v 1.0A and up, cable must be 20AWG or at least 0.75mmsq

5v must not go below 4.9v nor beyond 5.15v

Reference

https://www.raspberrypi.org/forums/viewtopic.php?t=224493

First things to do after installation of raspberry pi OS

Make sure the internet connection to update old packages by blind period between time to make official OS and current status.

# sudo apt update; sudo apt full-upgrade

In order to reduce sd-card usage down as much as possible, in my case these commands may be helpful after installation.

# sudo apt purge libreoffice* thonny* smartsim* scratch* sonic-pi* wolfram-engine* geany* mu-editor*
# sudo apt autoremove
# sudo apt clean

then, to fuse firmware is needed to improve the performance of your raspberry pi.

# sudo rpi-update