how to fit 2 gang lighting switch pls help....?

Does the upper landing switch also have a switch upstairs? Also, does the lower one have one? If so then the switches are 3 way switches, meaning they complete the circuit to turn on the light from either location. If you're replacing them then place the wires from the old switch to the new one in the same manner. Make sure to source out the breaker to remove power pefore doing so.I've linked a sample wiring diagram below. if it's a single switch operating a single light then it's straight forward one Line-side wire to the one terminal and the Load(light fixture) wire to the other.

It sounds like you have bought a 2 gang 1 way switch (Both switches operate two separate lights that are not connected), take the switch back and change it . A 2 gang 2 way switch can operate two separate lighting areas, these can be switched off at two separate switches ie 1 switch/ toggle could work the hall light and also have a second switch at the other end of the hall to turn the light off .The other toggle on the 2 gang switch would operate the landing light which has a second switch up stairs. These can also be used as one way switches using only 2 of the 3 terminals on each switch. So you may have a 2 gang 2 way switch, wired , 1 gang as one way the second gang as a 2 way. anyway it sounds like you bought the wrong switch.

When swapping an existing working switch (2Gang; 2-way) then just copy the connections. Otherwise an electrician would charge 50 quid to swap them for you...

Try here,hope it helps.