You can use Pega's OOTB activity work- .Reopen to reopen a resolved work object.
If you want to reopen to a specific stage you can use Work-.pyReopenToStage .By default, it shows the flow action to reopen the last stage in the flow action, with an alternate choice to change to any stage. You can customize Work-.pyReopenToStage.
Thanks, but these rules can be applied when we are in the context of the case we want to reopen.
In my scenario, we are in the context of another application, we try to leverage Pega API to send a reopen signal to the other case. For example we use PUT method of cases api to update another case. Can we also use Pega API for this roepening a case?