can I upgrade..I have a mid-2011 OS High Sierra 10.13.6

Hello rsmith,

Unfortunately you don't say which Mac model you have. If you mean upgrade from High Sierra then the answer is no, unless you have a Mac Pro and got the year wrong.

Install macOS 10.14 Mojave on Mac Pro (Mid 2010) and Mac Pro (Mid 2012) - https://support.apple.com/HT208898

Yes.

https://discussions.apple.com/docs/DOC-12081