How to fill up cell in Excel ?

select the empty cells with the cell above it and press F2 if text. this will copy the original one in all the selected cells under it. if you have numbers cells click on the cell then on the right side of it click on the plus sign and drag down. that will give u1,2,3,4 

lots of ways to quickly and easily do it 

Type the content you want to fill in the cell by using keyboard

By dragging the first cell that contains data to fill up the lower cells in the same column with the same data.

Ensure that the cell is not protecting from editing, it it is unprotected then select this cell and fill it with the data you want. you can drag the first cell that contains data to fill up the lower cells in the same column with the same data otherwise the first cell contains numbers, in case of containing numbers, the will change in a serial way between the lower cells

you can validate the cell to select data from a drop list or validate the cell to accept a specific type of data such as numeric or between two numbers or from a list