Pruning is best done in later winter (February/early March) to support the tree health. If you have branches, such as those in the photo, you can temporarily prop them up with 2 x 4 wood or ladders. If there is no way to prop up and the branch appears at risk for breakage you could choose to prune as a last resort. The link below gives information on caring for plums, including pruning methods.

https://extension.umn.edu/fruit/growing-stone-fruits-home-garden