While the 'Quick Create' option of building a forecast from last year's actuals gives you a duplicate of the previous year, using the 'Advanced Create' option allows you go into much further detail and alter those numbers.
You can add percentage increases or decreases to account categories, for example a 10% increase in income, or you could add a fixed amount increase or decrease, for example £10,000 addition to income.
If you wanted a bit more information, this help guide goes into the process of getting a forecast set up with this method!