Hello! It really depends on the context, but the sentence generally means "to not know how to do it." If you could provide the context, I could probably explain it to you better. "晚装发型不知道怎麼弄" means, to not know how to do your hair for a cocktail party. 弄 isn't a very formal word and is usually used in day to day conversations and casually. You can use it to replace a verb when you can't think of which verb to use at times. "方向盘撞歪了，不知道怎麼弄好" here, 弄 implies 修 or repair. "(My) steering wheel was knocked crooked, and (I) don't know how to fix it."

不知道怎麼弄= I do not know how/what to do with something 不知道=do not know , 怎麼=how/what.弄=to do or to deal with

弄 verb can be lots of verb 不知道怎么弄 i don't know how to deal with it

"不知道怎麼弄" 1、不知道怎么处理，如：这事太急，不知道怎么弄？ 2、不知道怎么修理，如：这坏表太先进，不知道怎么弄？ 3、不知道怎么整理，如：这资料太乱，不知道怎么弄？ 4、不知道怎么写，如：这资料太乱，不知道怎么弄？ 5、不知道怎么打扫，如：这房间太乱，不知道怎么弄？ 不知道怎么弄？意思取决于前面的那些句子的内容。