macOS 10.13 High Sierra is the highest version of macOS Apple supports on the 2011 Macs.
How to download and install macOS - Apple Support
Depending on how you use this Mac, you could install Linux Mint on it which gives you a current up to date OS (non-Apple) which gives you access to most popular third party web browsers (Firefox, Google Chrome, Vivaldi, etc.) and free open source software such as LibreOffice. Linux is not for everyone and it requires learning a new OS, but it is a great way to extend the useful life of older computers. You can even test drive Linux Mint by booting the Linux Mint installer & selecting the "Live" option....performance will be extremely slow, but it will give you a peek at what Linux Mint is like. Linux Mint will perform much better when installed to the internal drive assuming the internal drive is not worn out or failing.
Just make sure to first create a bootable macOS 10.13 High Sierra USB installer now while you still can in case you decide Linux is not for you on this old Mac.