Hi,

Pruning floricane raspberries like Latham is usually done in the fall when the plants are dormant. The tallest canes should be the canes that already bore fruit this year and won't bear fruit next year.

Cut them back on a dry day. See the information here:

https://extension.umn.edu/raspberry-farming/pruning-and-training-raspberries

These raspberries are tall by nature and next year you may want to stake them to prevent flopping. Good luck!