Did you add extra letters to your font, exported it and cannot see changes in an app that previously used it? No worries, it's probably just a matter of restarting your app:
On iPad:
You must Force Quit the app to restart it in blank state.
Swipe up from the bottom edge of the screen and hold to view active apps.
Swipe up on the app you want to kill.
On Mac:
Close & restart your app.
On Windows:
You cannot overwrite a font that is currently used by any other app.
Close & restart ALL APPS that may display your font.
Mind font cache problems
When you install a font on an operating system, or straight into apps like Procreate, it is copied and locked by the operating system (or the apps themselves) to ensure they show up quickly. A drawback is that even if you delete or replace a font file in your system font folder or in the app, you might still end up with the previous font version.
On Procreate:
Go to your iPad's Settings > General > iPad Storage > Procreate and press the button Edit above the fonts list to delete cached fonts, and Force Quit the app to ensure new fonts can be loaded without any interference with the older versions:
On desktop: Closing apps is usually sufficient, but if you don't notice any changes, restart your computer. Worst case scenario, you might ask Google to help you clear your system's font cache.
Mind duplicate font names
When you manually install fonts, it is recommended to delete any previous font that uses the exact same Font Family & Font Style names before installing a new one (remember that the actual font file name does not matter here).
Mind duplicate fonts in Adobe apps
Also look for duplicate font files on both your system font folder as well as the Adobe font folder:
On macOS:
Users/USERNAME/Library/Application Support/Adobe/Fonts
Tip: on Mac, press Cmd + Shift + G from the Finder and paste this line
~/Library/Application Support/Adobe/Fonts