Why does number of impressions decrease?

On August 12 it was 2,254
on August 13 it was 2,012

what does it depend on?

And how can I update my ad info? Just using the form create new ad?

 

Yes, you can't edit your ads, so you need to create a new one and either disable or delete the other one

 

thanks! may I ask one more question?
what for is the drop down list with figures from 1 to 15 next to

Disable
Enable
Stats

 

why impressions are dropping????

 

It depends on the sites your ads are being shown on......sometimes they get on a large site with high PR. In a few days you will probably see another spike upwards.
My sites impressions can go from 1k to 20k and back again, it's quite normal and averages out over time.

 

thanks for explanation