In the latest Sia version you can select your own password, but only when you initially set up the wallet. We are working to introduce a change password feature soon.
In the meantime, if you want, you could delete your existing wallet and download the latest version, then set up a new wallet and import your old seed.
To do this, click on the "Terminal" tab and type `wallet init --force`. This will delete your current wallet and generate a new one. Save your new seed and choose a custom password.
Then, use the import seed button to import your old seed into your new wallet.