At the top of the Overview and Insights pages you'll find two date controls.
Date range: choose the period you want to look at, for example This month or Last 30 days.
Compare period: optionally pick a second period to compare against. When a comparison is set, your key metrics show how they've changed versus that period.
One tip: changing the main date range clears any comparison, so set your main range first, then add the comparison.