It's very easy, you first need to remove the current time file
rm /etc/localtime
and replace it by the new one, you need to search the biggest city in the area you want
Here is the list : https://www.archlinux.org/packages/core/any/tzdata/files/
for the example let's take :
/usr/share/zoneinfo/right/Europe/Amsterdam
you simply need to do the command : ln -s /usr/share/zoneinfo/right/Europe/Amsterdam /etc/localtime
Replace the red text by the line you wish
Here are some examples :
for Cairo :
ln -s /usr/share/zoneinfo/right/Africa/Cairo /etc/localtime
for Berlin :
ln -s /usr/share/zoneinfo/right/Europe/Berlin /etc/localtime
And then you can check the time with the command : date