Q.Methley Plum
Lot of branches on my plum trees are dead. Some areas are OK. What do I do?
I would certainly trim away the dead material.
A soil test may be a good idea. You can then determine if the soil is the cause of the tree struggling.
Watering? Does the tree need more or less?
Are there signs of insects or fungus?
Here are some links to refresh you on the care requirements.
https://www.gardeningknowhow.com/edible/fruits/plum/growing-plum-trees.htm
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-pruning.htm
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm