how to change node edit path from node/[nid]/edit to node/[nid]/update? I have Pathauto which changes node/[nid] to having a path of [title]; I then use subpathauto to change node/[nid]/edit to [title]/edit.

But now how can I change the edit fragment? I want to change all instances of edit to update. so the URLs would be [title]/update instead of [title]/edit.

I cannot use arguments or wildcards in Pathauto, so I cannot specify a path like */*/edit to be */*/alter, but that is effectively what I would like to happen.
I think this is a legit request as long as you add an additional alias as opposed to overwriting system paths.

I would implement hook_node_insert() and then use path_save() to set your custom edit path on node creation. You will also probably need to make sure that this hook runs after the Pathauto module has set its path, so you can use the alias it sets as part of your additional edit alias.