Does current in a circuit change when it passes through a resistor?
It is known that current in a series circuit remains the same.

The current is the same at any part of a simple series circuit.  But that doesn't mean the current stays the same if you change the circuit.
An LED drops around 1.5 to 3V when it's working normally.  If you connect it straight across a 5V supply, an excessively large current flows, and the LED blows.
Put a 100 ohm resistor in, and the extra voltage is dropped across the resistor.  This will be somewhere between 2 and 3.5V.  Given I = V/R, a 100 ohm resistor will pass between 0.02 and 0.035A (or 20 to 35mA).  That's much better for an LED.