If you're comfortable with Android and locating older versions of the Fitbit app, look for 2.67.1. This app version just helped me update a new-to-us Fitbit we just bought, but had old firmware (5.86).
Locating sites that happen to have old versions of the Fitbit app, that's an issue. They aren't posting it themselves here, so you have to boldly go and locate it yourself.
If you're up to downloading the app from an online archive of your choice, you can delete the current app on your phone long enough to load the old alpha-papa-kilo file. Open the old app when it's ready and log-in. Pair your Fitbit Zip and sync/update your firmware on your ZIP (takes about 10 minutes). Then get rid of the old app and put the new app back on your phone when the firmware is up-to-date. It worked for me and we've be using Zips for... 10 years? Can't help it, I don't want a "smart" watch, I like my analog and as a former medic I always want a reliable second hand ticking away on my wrist for taking a pulse, not a need-to-charge weekly LCD device. Preferences, we all have them!
