The following describes how to stop nominating and retrieve your tokens. Please note that all networks on which you can nominate have a delayed exit period, called the unbonding period, which serves as a cooldown. You will not be able to transfer your tokens before this period has elapsed.
Step 1: Stop Nominating
On the Polkadot UI navigate to the "Staking" tab.
On this tab click on the "Account Actions" tab at the top of the screen.
Here, click "Stop Nominating" on an account you're nominating with and would like to free the funds for.
After you confirm this transaction, your tokens will remain bonded. This means they stay ready to be distributed among nominees again. To actually withdraw them, you need to unbond.
Step 2: Unbonding an amount
To unbond the amount, click the little gear icon next to the account you want to unbond money for, and select "Unbond funds".
Select the amount you wish to unbond and click Unbond, then confirm the transaction.
If successful, your balance will show as "unbonding" with an indicator of how many more blocks remain until the amount is fully unlocked.
This duration will vary depending on the network you're on and will typically be four times as fast on Kusama as it is on Polkadot.
Once this process is complete, you will have to issue another, final transaction: Withdraw Unbonded. Then, your transferrable balance will increase by the amount of tokens you've just fully unbonded.