There's a way to do it from GitHub's webapp.
Let's go through the following example.
To start with, open the repo that you want to update.
One can see the warning
This branch is 157 commits behind GoogleCloudPlatform:master.
On the right there are two buttons Pull request
and Compare
. Press Compare
.
As there is probably nothing to compare, press switching the base
A list of all the changes will appear and one can create a pull request by pressing the button Create pull request
Give it a title, let's say "Update repo"
And create the pull request.
Once the request is created, scroll to the bottom and press Merge pull request
.
Confirm the merge and that's it!