Revitalising a late 2013 iMac
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These 2013 iMacs require you to 'cut' the sticky tape keeping the screen on the iMac to get to the drive. This is pretty easy these days, providing you are careful and ensure the tape is properly separated and have the proper tools.
Anyhow, more information here. Spare parts can be obtained here or here. Don't skimp out on the tools, they are very useful.
If you've got family coming around for xmas and their old/slow iMac could do with an upgrade, have a go at doing it yourself. It's not too hard though somewhat fiddly (but follow the steps closely mentioned above). It brings a lot of 'tension'/'risk' as you are doing it and has a better feeling of accomplishment rather than doing a jigsaw this time of year... :-)
(Note: I understand many iMacs from 2013 follow the same upgrade process, but make sure you follow the appropriate guide on iFixIt when you do...)