How to replace, (the word how missing ) means many things.
but there NO HDD here, ok? its SSD already but pathetic in size.
but on any computer, old. (generic answer, more later)
1: we buy new SSD , HDD are old and slow, and we bet SSD.
2: then we install it, means remove old and put in new. (screws out then back in , all)
3: then we install the OS from scratch, there is no OS now, so must be installed fresh and new, (no OS stated by you)
Asus, All-in-One (AIO) A10 fake desktop it is.
there are man sub models of this PC.
here is the top of the list of 10 models. IUKH sub.
it's 6 years old.https://www.asus.com/us/supportonly/ET2221IUKH/HelpDesk_Manual/
this lame AIO came with (2 options)
16 GB (32-bit) or 20 GB (64-bit)
(drive space) pathetic is the word here , not any good for W10.
this toy grade PC has no service manual not even a full spec.
what it has inside I guess is a early form of M.2 SSD.
it does not have a real spinning (rust) HDD.
not at all. I bet its not bad the SSD, only too full.
16GB is hopeless, sized, windows live updates will kill that space dead.
Some of these PCs have a soldered down FLASH I.C. chip as seen 2009 era (and can not be replaced)
old: 2009? mSata device (mini-sata)
as tech and the lack of ASUS spec. sheets. or service manuals of any kind...
but but… first..... things ,first.....
we see if the 16GB is full. and fix that.(clean it)
if not full and boots ok, with free space (1/2 empty is best practice) do that now.... ask how.
if needing more space, SURE, we then open the AIO up, full only and find this silly 16GB wonder chip, to see if it can be replaced at all.?
if not we just scrap it this PC. (2013 made)
some of these cheap PC can not be upgraded.
but if there is a Msata card or real SATA DRIVE, yes.
1: find the 16GB device, it is NOT any HDD, but early SSD.
2: then try to upgraded it, (look first and tell what is there)
some other submodels (unstated by you )
do have a real HDD dead center, rear (case back off)
just below the fan, metal cover off that and here.
what you have , we will never know until you learn to post full model numbers.
et2221-xyz123 ( what is xyz123)?????????????????