@Domagoj_Hamzic To rollback to the previous activity you can use process-instance modification rest api. You need to pass set of instructions like to which activity to move and which activity to cancel.
https://docs.camunda.org/manual/7.15/reference/rest/process-instance/post-modification/
https://docs.camunda.org/manual/7.15/user-guide/process-engine/process-instance-modification/