If you are not familiar with Synology and their DSM OS it would be good to know that when it comes to updates, there is only one official way, forward. Meaning you can't downgrade DSM back to previous versions.
Now, this is not entirely true as there was an unofficial way to downgrade that is still valid but I can't stress enough how much this is NOT officially supported, and that you should do this only if absolutely necessary.
DISCLAIMER: the method you will follow below is unofficial not supported by Synology in any way. Any problems that you might have or negative outcomes are your responsibility and your alone. The author of this article will take no part in being accountable for these actions.
More info on this here: DSM 7 to DSM 6 downgrade (unofficial and unsupported)