how can i adjust project target duration?

read the tutorial of how to create a schedule... that will help

Hello,
A project target is only another project recorded in your P3 files and then shown on your layout when selected.
You need to open the project you use as a target, change whatever you want and it will appear with its modifications on the project you use.

If you choose to use the update target feature I would save a back up first. Make sure that you choose only the items that you want to have updated on the target file. It is easy to update more than you wanted to if you are not careful.