Do I need to resuspend FlexiTube siRNAs in siRNA Resuspension buffer?

You just need to add sterile RNase-free water (which is supplied with your siRNAs) to resuspend your FlexiTube siRNA. You do not need to perform incubation steps at 96°C or at 37°C. Your siRNA comes preannealed and does not need any further preparation prior to use.