Unlike your public key (your wallet address, always starts with the letter 'G') which can be shared with others to receive funds; the secret key (always starts with the letter 'S') is exactly that, a secret, and should not be shared with anyone. Your secret key (skey) which is given to you upon wallet creation is very important and should be stored in a safe place. Without your secret key, you are unable to restore your wallet should you lose your device, reinstall StashApp etc. It should never be given to anyone, not even the StashApp team. We will never ask you for your secret key.
That being said, let's show how to reveal the secret key so that you you can back it up if you haven't already.
From the Assets tab tap on account as in the image below:
From there tap on Wallet & security as in the image below:
Next tap on Reveal secret key as in the image below:
Your Secret key will become visible. If you have enabled Biometrics or Use PIN in Security options you will be prompted to unlock before the secret key is revealed. Next tap on the copy icon to copy the secret key to your clipboard, as in the image below:
Temporarily paste the secret key somewhere to be able to see the full key so that you can write it down and back it up.
Important note about storing your secret key.
Keep multiple back ups of your secret key. Keep it in a safe place where no one has easy access to it (anyone with access can restore and empty your wallet).
It is best to store your skey offline, written down on paper and locked away in a safe. If you are going to store it online, be sure to encrypt it in an archive with a very secure password.
Storing the skey in your note application is not advised.
There are multiple articles online about how to safely store your seed phrase. A search on a search engine will bring them up. Find a backup method that best suits you.