I'd keep it a secret except from the closest people to me. I'd be incredibly boring about what I'd do with it.
First I would pay off my mortgage and invest a chunk in "safe" investments - so shares in utility companies, funeral business - boring reliable investments - and property and land, across borders. All to try and guarentee I would stay financially secure long term for the rest of my life, and weather financial storms.
I'd help my immediate family financially (siblings and parents, and closest friends) but would not go over board - I'd make their lives better but not ruin them, and would aim to keep most of the money ready to keep helping for years to come rather than splurge out. And I wouldn't tell them how much I had so as not to ruin relationships.
For what I do for me I would think very hard. I'd probably not quit work immediately and I'd try not to ruin my life.
I'd probably look to travel but in bursts - either nice holidays and keep working (I like my job) or quit work and live 3 months at a time in places I've always wanted to be for a bit before settling down again.
Anything I do or buy I would do as someone "middle class". So I'd travel economy plus, I'd stay in decent hotal but not the most flashy, I'd buy a decent home but not a mansion (I don't need a 10 bed home, I'd just get a nicer version of what I have now - 3 beds but maybe detached and in a nicer area).
Basically I'd upgrade my life a little but I wouldn't go wild. I don't see the value in the conspicuously wealthy lifestyle - I'd see money as buying freedom but I wouldn't want to be wasteful, and I wouldn't want to be a target for criminals or leeches.
And the rest i'd start puting to good causes. That would probably be conservation charities, green charities, social projects I believe in. Id want to use it to create some kind of legacy even if anonymous - for me that would be something that meaningfully improved the world in some small but realistic way.
Basically I'd be very boring, stay anonymous and try and make relatively small but meaningful changes to my life and those I love.