Hi @Rabeb_Abid - for the first problem, you should try to explicitly model the return path. It is strongly recommended that you do not alter running processes via the API. That said, Zeebe supports a ModifyProcessInstance gRPC call that can move to another task. Another user asked a similar question yesterday, so it may be helpful to join the conversation in that topic: previous Task in camunda 8
For the second problem, I would again suggest explicitly modeling those tasks. For instance, before your user task, have a script task that does the calculation you need with FEEL and stores it in a process variable. Or it could be a service task that invokes a job worker you created that does a more complex calculation than can be done with FEEL alone. There are a few different ways to approach it, but you should do the calculations in its own task.