You get the picture. Select cell C3 and type an equal sign to begin the formula. Then click the tables Layout contextual tab in the Ribbon.
In its wisdom Word makes an educated guess about what you want the formula to do and places a formula in the Formula box.
SUMABOVE RIGHT would add all numbers that are above the cell and to the right. Go to Insert tab find Symbols group and click Equation button. You can click Write button and use your mouse or writing pad to write the formula here. From the top menu select Insert Object Create New.