Upgrade to cu6

Hi,
What you want to update. There are two parts of CU's (Application and Platform Hotfix).

For Platform hotfix you just need to reinstall navision from New Product DVD of CU6.

For Application Hotfix you need to merge the changes between CU4 and CU6 to the Custom Database. The Right Approach will be like -
1. Create a cronus database with NAV 2015 CU 4 Objects.
2. Try Importing Application part from the Product DVD to the New Created Database (on the Import worksheet, Filter on Field New Object Version >CU4 Build No).
3. Export the Filtered Object List into Excel.
4. This is the list of objects that have been changes after CU5.
5. From the Database that have CU4 Objects Export these objects as Text, then Import CU6 and again export these objects.
6. Compare these objects and merge changes on the Custom Database.

You can also merge these objects using auto merge utility available. Refer Here for Details.

Let me know if any doubts.