I have Macromedia Fireworks 6. What is the latest version and how do I upgrade?

Just save yourself a lot of time looking through all the lists, if you are not prepared to pay for a full Adobe subscription, and go the Affinity route. I used Fireworks since v3 and share the same feeling like you and a lot of people who are disappointed with what Adobe did to it, Affinity is by far and away the nearest thing, you have two options:

1) Buy Affinity Designer which has most of the features of Fireworks bar  a few and some extras features.

2) If you want to go for Fireworks on steroids you can buy Affinity Publisher, Photo and Designer and take advantage of something called Studio Link which allows you to use the features of all three Affinity apps effectively in one app. It is a very useful feature.

**I'm not affiliated to Affinity and no have vested interest, just my opinion as someone who has looked at all the alternatives...

Fireworks is discontinued, unsupported legacy ware.  It hasn't received any updates in a more than a decade.

If you have a paid Creative Cloud all apps plan and a computer that can still support it, Fireworks CS6 is the last version Adobe released.  Tick the box to Show older apps in your Creative Cloud Desktop App.  

Fireworks alternatives.
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- Illustrator CC: https://www.adobe.com/products/illustrator.html
- Photoshop Elements: https://www.adobe.com/products/photoshop-elements.html
- Photoshop CC: https://www.adobe.com/products/photoshop.html
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- Affinity Photo & Affinity Designer: https://affinity.serif.com
- CorelDRAW Graphics Suite: https://www.coreldraw.com/
- GIMP: https://www.gimp.org/
- Gravit Designer: https://www.designer.io/en/
- Inkscape: https://inkscape.org/
- Krita: https://krita.org/
- Paint.NET: https://www.getpaint.net/
- PaintShop Pro: https://www.paintshoppro.com/
- PhotoPea: https://www.photopea.com/
- Photo Pos Pro: https://www.photopos.com/PPP3_BS/Default.aspx
- Pixlr: https://pixlr.com/
- Vctr: https://vectr.com/

Hope that helps.

When you write "Fireworks 6" do you mean Fireworks MX or Fireworks CS6?

If you have MX, the only way to "upgrade" to CS6 is to rent the entire CC suite. Then CS6 becomes available as a legacy app which is no longer developed or supported by Adobe since 2013.

If you are running CS6: congrats, you are already on top of things and have the latest (read: last and final) version of Fireworks.