MicrosoftWord - UnDo

edit menu. Its the first item. place your cursor on a blank area of the toolbar space right click this will bring up a list of tool bars click "standard"
in my word 2003 the undo/redo arrows are located in the center portion of the tool bar.

from microsoft word help

Undo mistakes
On the Standard toolbar (toolbar: A bar with buttons and options that you use to carry out commands. To display a toolbar, press ALT and then SHIFT+F10.), click the arrow next to Undo .
Microsoft Word displays a list of the most recent actions you can undo.

Click the action you want to undo. If you don't see the action, scroll through the list.
When you undo an action, you also undo all actions above it in the list.

Notes

You can undo the very last action you took by just clicking Undo on the Standard toolbar.

If you later decide you didn't want to undo an action, click Redo on the Standard toolbar.

You should be able to undo by holding down the control key (ctrl, there are 2 of them, you can use either one, they're on the bottom row of the keyboard) and hitting the z.

If you want to use the icon, you need to add the icon to your basic toolbar. This is easy. Go under View (top of the page), then Toolbars, then Customize. You can select a complete toolbar to add (I believe undo is under Standard) or click the Commands tab, select Edit on the left and Undo on the right. Then drag the Undo command up to the toolbar.

Hi there.
An alternative would be to go to the Edit menu and select the Undo entry at the top.

I hope this helps.