On a fresh install: open the ‘Wallet’ tab of the UI and select “Load a wallet from a seed”. Enter your seed for both the seed and password fields. Password can be changed afterward via ‘Change Password’ in the ‘Wallet’ tab, but note this password applies only to this specific installation of SiaPrime (and never touches the blockchain).
On an existing install/wallet already loaded: either recover/sweep the seed (see above FAQ), or force the UI to switch primary wallet to new seed by opening the ‘Terminal’ tab and using the command ‘wallet init-seed –force’. As always, keep backups of seeds.