The basic choice you have is "rebase or merge": https://blog.sourcetreeapp.com/2012/08/21/merge-or-rebase/
Here on SO, a helpful Q&A is at:
Make another branch default?
The remainder of this response borrows from that page.
If you want to close the "feature-branch":
$ hg checkout default
$ hg merge feature-branch
$ hg commit
$ hg checkout feature-branch
$ hg commit --close-branch
The simplicity of the revert/commit approach mentioned there also has much to recommend it:
hg revert --all --rev ${1}
hg commit -m "Restoring branch ${1} as default"
where ${1} is (for example) the name of the relevant branch.