Hello Haruna,
You can't move the process back, because the current activities were already created in the Database. If for some reason it sometimes gets stuck in that point, even if you could "send it back" the problem would continue and it would stop again in that point. In order to fix this you have two options in my point of view.
1- Suspend the current running process and launch a new process with the data of the older one that you suspended.
2- You can change the property "Allow Skip" (in the Development Environment)for that activity where it is getting stuck, then you would be able to skip it (through Service Center). Skipping an activity you must be carefull, you need to check if the next activities don't need any data or the data wouldn't be affected by this activity that you
will skip. For example, if you skip an activity that interacts with the data that you will need in a following activity you can have the same problem that you're having or having bad bata problems.
Was this clear for you? Let me know if you need further help.
Regards,
João Pêgas.