You can quickly edit the tab stops of a document by clicking on the
top of the document, just below the ruler.  Left-clicking will insert
a left-aligned tab stop; clicking and dragging will move the tab.
Double clicking on a tab stop will pop up the "Edit Tab Stop" window
from the Paragraph Designer; you can specify left/center/right
aligning of the tab, as well as a leader sequence (such as "...")
that can be used.  

You can also edit tab stops by opening the Paragraph Designer (Format
-> Paragraphs -> Designer) and selecting "Basic."