• Underline the incorrect question tag and correct them.
• They have not lived here for decades,have you?
• She turned towards the bus stop,did she?

A question tag means that you are supposed to write the opposite(negative) of the verb that has been mentioned in the sentence. If the sentence before the question tag is positive, the question tag must be negative and vice-versa. But also remember to keep the the tense and the grammatical number of the subject (i.e. singular or plural) same.
In the given sentences, 
1st sentence - the grammatical number is plural (they), but the question tag talks about singular (you). Hence, have you should be replaced by have they.
2nd sentence -  Since the sentence is positive (She turned), the question tag must be negative. Thus, did she should be replaced by didn't she.