Whereas with the earlier iMac's the glass front was help on by magnets which resulted in a easier separation.
The Late 2012 and onwards iMac. The glass is bonded to the rear with high adhesive tape. To separate it, there is a tool you can buy which looks like a miniature Pizza cutter, which is slid between the glass and the rear. After you have separated it, you then need to remove the remnants of the old adhesive tape, and then apply the replacement high adhesive tape, this can be purchased online.
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It might be worth doing what joe-h2o has suggested before contacting Apple, especially if it isnt the amount for ram you have causing the slowness.
I presume your iMac has Mavericks installed, if you go into the Activity Monitor and click the Memory tab. The part you are interested in, is now called SWAP USED instead of Page Outs.
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