Flash is no longer developed for Linux, new versions are only available when integrated with Chrome. If you are using the Chromium browser, you can extract the Flash plug-in from Chrome and use it. If you have Firefox, to get the latest version you will need to use a different browser. If you have Chrome and if it's up to date, you'll be fine.
Steps
Method 1 of 3: Chromium

Step 1. Open the Ubuntu repository
This application opens from the system task bar.

Step 2. Click on the Edit menu and then select Software Sources

Step 3. Click the Ubuntu Software tab

Step 4. Click the Restricted software for copyright or other legal reasons (multiverse) box
Then click on To close.

Step 5. Wait for the repository to update its sources
This may take a few moments.

Step 6. Look for “Pepper Flash Player” in the software source
Download the extension for the browser.
The proposed software package is called "pepperflashplugin-nonfree", but this extension, despite its name, is free

Step 7. Open a terminal
You can start it from the taskbar or press Ctrl + Alt + T.

Step 8. Type sudo update-pepperflashplugin-nonfree and press Enter

Step 9. Wait for the installation to complete
This may take a few moments. When the installation is complete, your computer name will reappear in the console. Type exit then Enter to close the terminal.

Step 10. Restart your browser
The Flash Player extension is now installed for Chromium.

Step 11. Periodically check for updates
When an extension is installed this way, subsequent updates do not happen automatically. You will need to manually check for Flash updates on a fairly regular basis.
- Open the terminal.
- Type sudo update-pepperflashplugin-nonfree –status then press Enter to search for available updates. If the updated available version has a higher version number than the installed version, then an update can be downloaded.
- Type sudo update-pepperflashplugin-nonfree –install and press Enter to install this update.
- Restart your browser to finish.
Method 2 of 3: Chrome

Step 1. Keep Chrome up to date
The Flash Player extension is built into the Google Chrome browser and you won't have to do much for it to work. Just keep the browser up to date and the extension will work fine.
If the Flash plugin is "broken" in the browser, try uninstalling it and then reinstalling it. There shouldn't be a problem
Method 3 of 3: Firefox

Step 1. Change your browser to Google Chrome or Chromium
Adobe no longer supports development for Linux with the exception of the Pepper Flash extension for Google Chrome. This means that the plugin for Firefox has become obsolete and no longer receives any improvements, except for a few security updates.
If you want to install an older version for Firefox, do the following

Step 2. Do CTRL + alt = "Image" + T (at the same time)
Otherwise, press the "super" key (the Windows key) and type "Terminal." The Terminal should appear.

Step 3. Write "sudo apt-get install flashplugin-installer"

Step 4. Enter your password for sudo
You cannot see any asterisks, but the password is still entered.