The newer Asus motherboards and bios can use both, a traditional 2.5 SATA 3 and the new M.2 types SSD drives; singularly or together.
Yes, you can upgrade the existing 2.5 HDD with a Crucial 2.5 SATA 3 upgrade as shown on the CRUCIAL website. More importantly, you can ADD an M.2 SSD and move the OS/APPS there and have a really fast boot drive; leave the old drive as a data drive.
You have two choices for use of the M.2 slot depending on the bios. (1) Install either an M.2 slot NVMe SSD drive (if you have a newer bios) or, (2) an M.2 type SATA 3 SSD drive which is slower but still many times faster than the existing 2.5 HDD.
We added the following M.2 SATA from Amazon because we had an older bios installed:
(1) Crucial MX500 250GB 3D NAND SATA M.2 Type 2280SS Internal SSD - CT250MX500SSD4
(2) We kept the existing 2.5 2T HDD as a data drive. I would prefer to replace the HDD with a 2.5 SATA 3 drive and make the HDD into an external backup storage drive, but a large 2.5 SSD is expensive.
It is clear that Asus has not properly detailed the existence of the M.2 slot on this motherboard or other similar motherboards. I have seen them offer these boards in different configurations through mass retailers where they use the slot and drive along with upgraded memory to sell the same machine at a higher price point. It is my opinion they do not want us to buy a discounted version and then upgrade it ourselves using 3rd party parts.