To open a database you will need to open it in the Viewer Window. There are two ways of doing this described here.
Now you should be able to see your database. If the database is quite small use the '+' buttons to add more columns and rows. This will not change the database in any way as the added cells are all empty and will be ignored and not updated to the database.
Note: Any changes you make using the methods bellow will only effect a local copy of the database on your computer. To update the online version of the database you need to press the 'Update' button in the top right of the window which appears when the database has been changed.
Important: Seriously, don't forget the point above. After speding time editing the database, it is so easy to just close the window forgetting to apply the changes.
You can change a cell value in a similar way to Microsoft Excel. Click on the cell and there is a text field at the very bottom of the window where you can enter the cells value. Once you have selected this text field once it should remain selected as you click on other cells to edit them.
Shortcut: With a cell selected you can press the Return key to select the cell bellow. In the Unity Editor this shorcut isn't very responsive so you may have to press the key twice.
To clear all cell values or delete all cells from the database press the black button which is in line with the row numbers and column numbers. This will show you a context menu allowing you to clear/delete all cells.
To perform Row or Column operations such as Insert, Clear and Delete, press the button which displays the row/column number of the row/column you want to perform the operation on.