>> If there are 2 or 3 change of trains, will the reservation fee of CHF 5 applicable on each sector or it will be from starting point to destination?
Yes, a reservation is specific to one particular train. If you change trains and want a reservation on the next train, you need to buy a separate reservation.
However, reservations are not compulsory within Switzerland, except on the trains specifically catering for tourists, i.e. the Glacier Express, Bernina Express and Gotthard Panorama Express.
>> was mainly concern with my first day travel from Zurich Airport to Interlaken with a change of train at Bern. IC 61 975 Direction Interlaken Ost Bern 4:04:00 PM Interlaken OST 04:57:00 PM
That particular departure from Bern is likely to be crowded with commuters, due to the time of day. If I were taking that train, and travelling with luggage, I would make a reservation. It is unlikely you would have to stand all the way to Interlaken Ost, as commuters will get off along the way, freeing up seats, but I hate standing at any time when I want to enjoy the scenery!
If you have enough time, you could consider travelling via Luzern/Lucerne and over the Brünig Pass, which takes about an hour longer. This is part of the Golden Pass route and is much more scenic than the route through Bern.
I would recommend reservations on that train as well, due to the time of day.
>> Also are the reserved seats marked as reserved?
Yes, there is an electronic display above the seat, showing the towns between which the seat has been reserved.
Something else to look out for are seats set aside for last-minute reservations.
These seats have ggf freigeben on the electronic display showing reservation details above the seat. This is short for gegebenenfalls freigeben (vacate if need be), which means that the seat will need to be surrendered if anyone turns up with a reservation for it.