I wouldn't be too concerned about taking it to Best Buy. It's a valid solution and corrupt firmware isn't generally the basis for a replacement device. Things do go wrong sometimes but can easily be fixed.
First thing I would recommend you try is clearing the system cache. Ideally after a system update the device should be reset but it's a hassle for most so clearing cache may help with any gremlins in the system.
Turn off your Samsung Galaxy S6 completely. Then you have to press the following keys to turn on the phone on again: Power On/ Off Volume + Home button If the Samsung Galaxy S6 vibrates once, release the power on/ off button but keep the other two keys from the key combination pressed until a small menu appears on the screen. Now mark the entry "Wipe cache partition" with the volume down key. With the power on / off button the entry can be selected selected and the process gets started. This usually takes only a few seconds. Then the previously displayed menu appears again. Now select the first entry with "Reboot system now" and run the command again with a press of the power button. Your Samsung Galaxy S6 will now reboot.
If you still have issues then you will need to go to a Best Buy. Your data should be safe and contacts etc stored on your Google account are always there as a backup.
You can manually back up using Samsung Smart Switch which is here
Samsung Smartswitch | SAMSUNG UK
When you contact the phone to the PC you will need to swipe down the notification bar and select the MTP connection option.
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