you really need to hire a tape and bedder person to fix this. NOrmally you have to score the affected area and actually start over with the process and this time do it correctly.
Now if its a small pull away, sometimes you can inject some caulk behind the tape and flatten it out again and it will stick, but normally this does not work for long. Your drywall has shifted and the tape has pulled loose.
I have even crammed caulk in behind the tape, then use pins to keep it stuck to the wall then when dry remove the pins and then paint over the area. But, again this is a band aid approach.