Hello Roger, 

Thanks for reaching out! The UMN Extension has a few very helpful articles about pruning apple trees: 

https://extension.umn.edu/fruit/growing-apples (scroll down to "Pruning and Training Apple Trees" section)

https://extension.umn.edu/yard-and-garden-news/nows-time-prune-apple-trees

Pruning is best done in the late winter or early spring before growth begins. If you'd like, you may consider thinning the fruit (see the first link above for more information). This can produce larger, higher quality fruit and should be done in late June or early July. 

Good luck!