If your later means in other parts of your code outside this method, then you have to declare a global variable at the class level and assign it to the value of FileName
Public Class Form1
Dim savedFile As String
Public Sub Form_Load(.....)
End Sub
Public Sub ButtonUploadFile_Click(....)
if savedFile.Length > 0 Then
.....
End If
End Sub
Public Sub ButtonSave_Click(......)
Dim saveFile As New SaveFileDialog()
saveFile.DefaultExt = "*.rtf"
saveFile.Filter = "RTF Files|*.rtf"
If (saveFile.ShowDialog() = DialogResult.OK) AndAlso _
(saveFile.FileName.Length) > 0 Then
savedFile = saveFile.FileName
rtb_Output.SaveFile(savedFile, RichTextBoxStreamType.PlainText)
End If
End Sub
End Class