This won’t be possible as you can only edit elements in an open transaction and Dynamo is designed to open and commit transactions on the active document (since all transaction objects require a document as an input), which means if you want to open a transaction on an external Revit file like in your case, you need to use Python or C# and create your own transactions using the link document object as the input to the transaction.
In addition, just like in Revit, it’s not possible to open a link if its host document is also open without getting the following warning. While its easy to dismiss this dialog using C#, the underlying rules cant be changed, i.e it would still need to unloaded even if it was automated:
In theory, working with these rules, it would be possible to automate updates to links by: dismissing the warning message, accepting this restriction and unloading the link, performing the update to the link, save, reload in the active document. The user wont even realise that the link has been unloaded, although if you have rooms or other elements bound to the link its all going to become un-associated, which creates a huge problem (unloading any file with constraints will cause this - as stated Dynamo is bound by the rules of Revit, it doesn’t create its own rules or behaviours!).
Option 2 and the best option (with a but) would be to set up worksharing on the link. In this scenario, it would be possible to create a local copy of the link, make edits to that copy, then synchronise back to Central. Finally, reload the link in the active document, and voila, you’ve just updated the link! Again, if this is all automated, then the process can operate without the user realising all of the steps going on behind the scenes - all they will see is the link updating.