In order to bond more funds to an existing stake navigate to Network --> Staking --> Accounts Actions on Polkadot-JS UI.

Once there, click on the three dots on the very right and select Bond more funds from the menu. (If this option is inactive, scroll down to the bottom of this article.)

On the following screen, enter the additional amount you want to bond. Remember not to stake all of your funds, but leave a small amount to cover any potential transaction costs.  

Then click the Bond more button. 

Confirm the transaction on the next screen.

That's it! You have now increased your stake!