Not a stupid question! Hey, if you don't know, you just don't know, but that does NOT make you stupid! Now, if you had been using Access for 10 years and did not know this, then I could at least say you were in the wrong line of work.
OK, here is the answer. I don't know of any wizard to help you change a report. But, if you open the report in design view, then using the menus, use menu View, option Field List, you will find a list of all the fields you can use on your report. If the field you want is there, use it. If it is not there, then you should have a query as the Record Source for this report. Open the Properties window for the report, click on the Data tab, and the click the elipse button "..." on the far right hand side of the Record Source property. If the button is not visible, click on the Record Source property line and it will become visible. After clicking the button, you will be presented with the Query Design Grid. There you can pick the field you need from the table shown in the top of this Query Design Grid and drag it into the bottom portion. Now you can go back to the report and use this newly selected field.
Hope this helps,