The keytop clips onto a small expanding frame over the key contact (in the centre). Place the keytop over the expanding frame, ensure alignment with other keys then press down firmly till it 'clicks'. If this does not work the small frame piece may be damaged, check it for broken parts near the corners. If damage is evident look for a 'faulty' keyboard to obtain the parts. Keytops and other bits are not usually available to purchase, only replacement keyboards! expensive!
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