I got talking to a friend the other day about Yo, the app where you can send your friends the word "Yo." It's nominally still around, run off donations, but the SSL certificate for the API server has been expired for a little while, so the app doesn't work anymore. Not to worry, though, that's something we can fix by patching the APK pretty quickly.
First, download Yo on the Play Store. Then, download APK Extractor, and use it to download the APK off your phone (you'll need to get it onto your computer somehow, I emailed it to myself). You should have a file called Yo_base.apk.
Next, install apktool, and use it to decompile the APK:
apktool if Yo_base.apk
apktool d Yo_base.apk
This should make a directory called Yo_base, which you can edit however you want. I changed https://newapi.justyo.co to http://newapi.justyo.co in res/values/strings.xml, but you could also make other changes as well. Once you've done that, recompile the APK like so:
apktool b Yo_base
Now there should be a Yo_base/dist/Yo_base.apk file, but it's not signed, so we can't use it. Signing it isn't too tricky though. Using the keytool and jarsigner tools that come with the JDK:
keytool -genkey -v -keystore my-release-key.keystore -alias alias_name -keyalg RSA -keysize 2048 -validity 10000
jarsigner -verbose -sigalg SHA1withRSA -digestalg SHA1 -keystore my-release-key.keystore Yo_base/dist/Yo_base.apk alias_name
It'll ask you to make a password and enter your name and things, I don't think it really matters what you choose. Once you've done all that, you can move the Yo_base/dist/Yo_base.apk file to your phone, click through all the fuss that Android makes about running a unsigned APK, and start Yoing away! This also works for other apps just as well :)