In the item window open Change Field Values... window from the Items menu.
The Replace operation is a Find/Replace. In the Item window mark all the items you would like to use a Find/Replace on.
Select the Field you want to find something in e.g. Description and set the Operation to Replace. Search for what you type in the box inside the Operation section and replace with the value entered in the value box.
In the picture below you can see how to look for the letter “L” in the “Description” field and replace all Ls with “M” for any items marked.
To read more about the Change Field Value window check out the Online Help Manual.