declare your variable under the class of form
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Dim rom As New OpenFileDialog
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After in a button or something for open the rom
rom.Filter = "Open Rom (*.Gba)|*.Gba"
If rom.ShowDialog = Windows.Forms.DialogResult.OK Then
Whatever you want

end if
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For save Rom i use Filestream to writebyte to determined offset or read byte

Read this:
Classe FileStream (System.IO)
Metodo FileStream.Read (System.IO)