How can I fix this bad drywall patch
Start out by sanding the seams and lumpy areas to determine if it's just a lousy taping job. If so, rip off the old tape and redo. If the drywall is droopy, you'll have to attach it to the joists with more screws. If the paper is separating from the drywall surface, you'll have to replace the drywall with some new drywall.