To the new Microsoft Edge.
Be warned, you'll probably find it easier to reinstall your favourite browser and set it as default than to try and make this change in Windows 10 yourself. As the deal with the EU has now expired, MS is under no obligation to make this easy for you. Because Microsoft now has less than 50% of the Browser market, the EU won't be able to pull that trick again either....
Personally I won't be using Edge as it is one of those "metro" apps, now being called "The modern UI" apps.
I have my own name for these Windows8 apps. Crapps. Because they almost all are.
So if you don't use IE and don't want to use Edge, then reinstall your browser of choice. If you use IE, then the old IE is still there, you'll need to go find it and stick it on the desktop and the task bar.
I've only upgraded one machine to 10 so far. It went pretty smoothly and actually kept more of my apps than prior Windows8 "refreshes" did. I'll be upgrading others as time goes on, including my main laptop. But I need to get my virtual server mess sorted out before that. I'm almost at the data extracted stage. Then I'll have to rebuild it and copy all the data back... Then back it all up properly.
At least another 2 weeks before I can really focus on Windows 10...