Get mut value from HashMap, edit it, and insert it back into HashMap

Question

I have a HashMap

HashMap<String, some_struct>

struct some_struct {
   id: i32,
   namespaces: Vec<String>
}

I want to be able to get some_struct from the map, push a String into namespaces and insert the new some_struct back into the map. Is this possible? What would it look like?