Changing column character between 2 workbooks

[chemhany]

I have 2 workbooks each of 31 worksheets. The 1st one contains data of which I want to calculate the average in the 2nd workbook separately for each worksheet. I used the function
=AVERAGE(INDIRECT("'C:\Users\lab9\Desktop\H\shift\[AN (32).xlsx]"&SHEET()&"'!B$7:B$18"))

How can I change column character, i.e, I want to change B-character in B$7:B$18 when dragging to the next cell to refer to column C ??

Thanks