Question

how does electronic configuration in atoms changes from top to bottom and left to right

Answer 1

on moving left to right 1 electron is inserted into the shell of the elements.
the order is K L M N
K - TWO ELECTRONS
L - 8 ELECTRONS
M - 18 ELECTRONS
N - 32 ELECTRONS
....and so on
on moving down in a period
first two ELECTRONS are added then 8 ELECTRONS are added again 8 electrons are added then 18 electrons are added and at last 32 electrons are added.
note - all the electrons are added in period wise order.