If its a small dissectum (Acer palmatum dissectum) variety, its not usual to prune much, but any pruning or reshaping that needs to be done should be between end of July and January. The trouble is, its mild, and the sap may already be up this year, in which case the tree will bleed badly. You could try cutting one stem to see if it leaks and if not, chance the rest. Cut just above a leaf node or bump on the stem with an angled cut.