By default, your 29 word Sia seed is your Sia-UI wallet password. But you can also set a custom password to unlock your wallet, and there are pros and cons to each. Let's take a look.
Sia-UI, downloaded and installed.
Your Sia-UI wallet set up.
This means you set your wallet password to a totally custom phrase of any length.
Go to the Wallet tab in Sia-UI.
Click More in the upper right corner.
Click Change Password. Sia-UI will ask for your current password. For current password, enter whatever you're using to unlock your wallet now. You can then enter your custom password.
That's it! You've changed your Sia-UI password.