The HDD-trays can be released through unscrewing the thumb screw in the right corner and then pull it straight out. These trays can support either a 3.5" drive, or a 2.5" drive. Below, you can see an image displaying how to mount a 2.5" drive to the SSD-brackets behind the motherboard. These have the same kind of thumb screw mounting as the above HDD-trays, but in the top instead of to the right of it.
Below, you can see an alternate place for mounting the 2.5" drives. There are mounting holes on the top of the PSU shroud for mounting them if you want to display them, or want to add additional trays (Purchasable separately. Product page for SSD-trays)
We also have an additional separate product we call a MultiBracket which lets you mount additional 2.5" or 3.5" drives on to unused 120 mm fan slots in our cases. If you are in need of additional HDD's in your case, this could be an alternative (click here for product link). Note that it could potentially lower the case airflow though depending on where you mount them.