Do you have access to the original solution that was used to create the report(s) you wish to edit? That would be the best/easiest starting point.
If not, there are a number of ways to get the .rdl from the Report Manager (ssrs website/administration portal). If you go to the properties of the report and click the edit link on the general tab you'll be asked to open or save the .rdl Save it to your machine make your edits and then deploy it to your SSRS instance. This works ok if you have 1 or 2 to edit.
IF you no longer have access to the original source and would like all of your report definitions so that you can edit and re-deploy them as needed, check out the Reporting Services Scripter available at http://www.sqldbatips.com/showarticle.asp?ID=62. It provides you a fair amount of options for downloding the rdl's including permissions and some other things. It even will download them and put them in an appropraite solution which you can then open with VS.
-Luke.
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