Find the appropriate option in the manager settings or type in the console:
sudo pacman-mirrors -g
It is worth doing it quite regularly (I personally do it once a month), because sooner or later the problem will return.
Or the graphical interface of mirrors (command from the console)
sudo pacman-mirrors -id
If we remove the letter d (default) from the parameters:
sudo pacman-mirrors -i
then the graphical interface is launched, with the possibility of selecting specific mirrors, but depleted according to the settings in the configuration file.