Select the cell where you want the vertical text
Right-click and choose Format Cells
Go to the Alignment tab
Under Orientation, set the text angle to 90 degrees or -90 degrees
Click OK
Or use the Home tab
In the Alignment group, click Orientation
Choose Vertical Text or Rotate Text Up/Down
Adjust row height and column width if needed
Use Wrap Text if the vertical text needs more space
For a single letter per line, type each letter on a new line with Alt+Enter