@Anu5619
Normally, when I do an alteration request, it shows me the change in price. Unless it specifically states that it's an extra fee, that doesn't mean that is a specific charge for changing the reservation. It's the difference in total cost from the original reservation to the new one.
This increase could either be, as @Helen3 suggests, because the new dates are priced higher than the ones you originally booked, or because Airbnb is charging you higher fees this time. The Airbnb guest fee is not a fixed percentage and varies. .
One thing I have read here on the CC from other hosts is that Airbnb sometimes charge a lower percentage fee for new or newer guests than regular users. I don't know if that is what is happening, but it could be that, because the booking has already been made, they ramp up the fee when it is altered. That's just a theory of course. I have no proof, but several times when I have tried to do an alteration request for a guest, I noticed the fees jump up a bit in the final calculation