Following code snippet can be used to change the values of a web part property in an existing SharePoint website:

using (SPSite spSiteTest = new SPSite(“SiteURL”) 

  using (SPWeb spWebTest = spSiteTest.OpenWeb()) 
  { 
    SPWebPartCollection webparts = spWebTest.GetWebPartCollection("WebPageURL", Storage.Shared); 
    
    for (int k = 0; k < webparts.Count; k++) 
    { 
      //get reference to webpart 
      Microsoft.SharePoint.WebPartPages.WebPart wp = webparts[k]; 

      //check webpart Title to find webpart whose value is to be changed 
      if (wp.Title == "TitleOfWebPartWhoseValueIsToBeChanged") 
      { 
        //set new property values of the webpart object 
        wpNew.Width = "600"; wpNew.Title = "New Web Part Title";

        //Call SaveChanges method of SPWebPartCollection object 
        webparts.SaveChanges(wp.StorageKey); 

        //update spWeb object 
        spWebTest.Update(); 
      } 
    } 
  } 
}