I seen still a lot of expats dont know what exact documents they need to prepare and where to go to renew Z visa.
here is a list and the right place to renew your visa,
hope it helps.

step 1：劳动局
Labor bereau（5个工作日）
1.新用人单位聘用申请报告
Employment application (reason, position, stamp)

2.前单位离职证明原件
Previous job release letter

3.新用人单位劳动合同复印件盖章
Employment Contact photocopy (stamp)

4.个人中文简历一份
Chinese resume

5.学士学位学历原件及翻译件
Original bachelor degree certificate and a photocopy, a translated version with translation company stamp

6.提供就业者在中国境内2年以上的工签签证复印件或者海外公司2年工作经验证明原件
2 years working experience certificate / 2 previous working visa photocopy

7.本人护照原件，住宿证明，照片2张
passport, registration paper, 2 photos

8.北京就业证原件或者外地就业证注销单或者迁移单
Work lisense / work lisense cancellation receipt

9.新用人单位营业执照复印件盖章，批准证书/社保登记证
photocopy of duplicate business licence, approvement certificate

10.就业证登记表盖章
Work lisense registration form (stemp)

step 2：出入境

PSB（5个工作日 5 working days）

11.变更后的就业证原件
Work lisense

12.护照原件，住宿证明，照片1张
Passport, registration paper, 1 photo

13.签证及居留许可申请表盖章
Application form of residence permit (stemp)

14.公司营业执照复印件盖章
Photocopy of duplicate business licence (stamp)