To merge one branch into another, such as merging "feature_x" branch into "master" branch:
git checkout master
git merge feature_x
This page is the first result for several search engines when looking for "git merge one branch into another". However, the original question is more specific and special case than the title would suggest.
It is also more complex than both the subject and the search expression. As such, this is a minimal but explanatory answer for the benefit of most visitors.