See the documentation here for the first optionL https://docs.mendix.com/refguide/version-control/#merging-branches
You can use the merge or advanced merge for this.
2 is not without risk. But if you keep in your model directory the hidden .SVN and .mendix-cach delete all the rest and copy the files over from your branch model directory to your main model directory and do a commit your branch has then become the main line. And if you are still on svn you could also use third party tooling to do this. Open the repository browser, delete trunk and move your branch to trunk.
For the 2 part do make sure you have crated a backup(s) so you can revert to that one if all goes wrong.
Regards,
Ronald