What do I do about a broken screen replacement (flickering while unlocking)?
The quick answer is that there isn't really much you can do on your end.

What can I do?


You could take apart the phone and attempt to fix the display manually, but by no means does this guarantee a fix (or even an improvement). By doing this, you would also run the risk of damaging your device further.
If Apple replaced your screen when it was broken, take it back to them and let them know it was not fully fixed. They should fix it again at no charge (if it was serviced within the last 90 days).


  Apple covers defects if they arise from authorized service for 90 days or the remaining term of your iPhone warranty, whichever is longer.

If a third-party dealer replaced your screen, take it back to them. Generally, third-party iDevice-fixing stores will repair it a second time, if they did not correctly repair it the first time (in order to keep your business).


Will my warranty be void?

Your AppleCare Coverage does not apply to:



  damage caused by accident, abuse, misuse, liquid contact, fire, earthquake or other external cause


  damage caused by service (including upgrades and expansions) performed by anyone who is not a representative of Apple or an Apple Authorized Service Provider (“AASP”)


  defects caused by normal wear and tear or otherwise due to the normal aging of the Apple Product, or if any serial number has been removed or defaced from the Apple Product.



What are Apple's Prices?


With AppleCare Coverage (screen replacement)


iPhone 6 - $109
iPhone 6 Plus, iPhone 5s, iPhone 5c, iPhone 5 - $129




  If your repair isn’t covered by Apple’s One Year Limited Warranty, AppleCare+, or AppleCare Protection Plan, your iPhone might be eligible for out-of-warranty service.



Out-of-Warranty (any repair)


iPhone 6 Plus - $329
iPhone 6 - $299
iPhone 5s, iPhone 5c, iPhone 5 - $269
iPhone 4s - $199
iPhone 4, iPhone 3GS, iPhone 3G, Original iPhone - $149



Note: An out-of-warranty battery replacement is $79, but only available if the battery fails Apple’s diagnostic tests.

What is AppleCare?


  Purchasing AppleCare+ extends the hardware coverage to two years from the original purchase date of your iPhone and covers two incidents of accidental damage, each with a $79 service fee plus applicable tax. AppleCare+ also includes Express Replacement Service so you can get a replacement before sending in your iPhone.
  
  If you didn’t buy AppleCare+ with your iPhone, you can buy it within 60 days of purchase. Take your iPhone to an Apple Retail Store or call Apple. We’ll verify the purchase date and confirm that your iPhone has no existing damage.


What are Apple's Repair Terms?


  For all service orders, Apple warrants to the extent permitted by
  law that (1) services performed will conform to their description for ninety (90)
  days from the date of service, (2) except for batteries described in the subsection
  below, all parts or products used in service will be free from defects in materials
  and workmanship for ninety (90) days from the date of service, and (3) batteries
  installed as part of Apple’s battery replacement service for Apple portable Mac
  computers will be free from defects in materials and workmanship for one year
  from the date of service. This warranty is an express limited warranty. If nonconforming
  service is provided or a defect arises in a replacement part or product
  during the applicable warranty period, Apple will to the extent permitted by
  applicable law at its option, either (a) re-perform services to conform to their
  description (b) repair or replace the part or product, using parts or products that
  are new or equivalent to new in performance and reliability, or (c) refund the sums
  paid to Apple for service.