The Data menu allows you to enter up to 50 pairs of statistics data. The Memory is shared between the data and cashflow areas, the 20b has 50 slots for pairs of statistical or cashflow data in total. If you try to enter the STATS menu before you have entered any data, the 20b will enter the Data menu for you. Numbers are entered, pressing [INPUT] in between them. Once your data is input, enter the [Shift] [Stats] menu.
To review or edit the entry by pressing the [Shift] [Data] and use the the [Arrow Up] and [Arrow Down] key to find the entry. Simply enter the new value and press [INPUT] to update the entry.