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It depends if your iPhoto version is recent enough for App Store to recognise it. If you can find newer update from Apples download site https://support.apple.com/downloads/iphoto App Store should detect it and offer latest El Capitan compatible update from updates tab.

If that doesn't work and you have another Mac with iPhoto installed you could try updating it, both Macs have to use same Apple ID or it won't work! Sometimes it works and sometimes it doesn't, I am not sure why.

Currently Photos is very limited compared to iPhoto especially when it comes to organising pictures and it may not suit your needs. In that case might be good idea to look for a third party replacement.
 
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