Thanks for the query.
The numbers in odd places are being decreased by 1 and the numbers in even places are being increased by 1.
In the first place it is 9 so in the third place it's 9 - 1 = 8 and in the second place it's 11 so in the fourth place it's 11 + 1 = 12.
So, the series will continue like this -
In the fifth place the number will be 8 - 1 = 7 and in the sixth place the number will be 12 + 1 = 13.