how to remove 21" iMac stand

It cannot be removed.

If you were hoping for VESA compatibility, that would require the purchase of a

specific model design from the Apple Store of the iMac from Late 2012 and later,

if a new model was desired. The online store shows the product, so a standalone

retail store or authorized Apple reseller, could get one, if not in their stock.

As stated, standard model with a stand attached already is

not able to have it removed or upgraded to another mount.

Here is a support article about these later models.

•iMac (Late 2012 & later) VESA mount compatibility:

http://support.apple.com/kb/HT5619

You'd have to choose to buy one with VESA bracket

& these do not appear to have a mount at all, those

are available as separate purchase. You choose one.

{Some earlier models a few years before had options

as mentioned in researched Support database articles

and if yours was a mid-2007 or so, it may have been

able to use a special Kit to change it to VESA. See

the manual if it is an older intel-based iMac series}

Hopefully this helps clarify that matter.

Good luck & happy computing! 🙂

Can you point out where this option is? I do not see it in the store checklist.

Not sure of online stores outside of the US and UK (both list them)

but here is a link to a current page in the Apple Store US...

•iMac - Buy iMac with Built-in VESA Mount Adapter - Apple Store (US)

http://store.apple.com/us/buy-mac/imac-vesa

{Said by some on introduction, to add approximately $40 or so to regular

price and early 2013 third-party press releases said the mount was extra.}

"Stand not included, desk/wall mount sold separately." -- imac apple store.

There likely is an option to buy some kind of mount in the custom configuration

section when one places an order through the retail online store page.

Good luck & happy computing! 🙂

•PS: some older iMacs could use a conversion kit, a similar kit was available

for Cinema displays to put them on a wall.. here's a link to the older accessory.

http://store.apple.com/us/product/MD179ZM/A/vesa-mount-adapter-kit-for-imac-andl ed-cinemaor-applethunderbolt-display?