What is the structure of your database? Do you have one record for each day? For each week? More than one for each day?
The basic method is to use a calculation field that computes the date for the preceding time period to be used with a relationship that then matches to only the records from that preceding time period that you want to summarize to get a value for your change calculation.
But the details depend on the records in your table as per my questions at the start of this post. Often, you need yet another calcualiton field that comptues a common value for every record that falls in that time period.