Open Wallet, choose Funding or Spot, find the asset and click Transfer.
Choose From account and To account (Funding or Spot). The arrows button swaps them.
If you have sub-accounts, also choose From user and To user (Main account or Sub-account). Signed in as a sub-account, From user is the sub-account itself, and under To user you can choose Main account.
Enter the amount in Amount and click Transfer.

From a sub-account's balances: in Wallet choose the sub-account in the Account list, then click Transfer out (from the sub-account to the main account) or Transfer in (from the main account to the sub-account) next to the asset. The form opens with the sub-account already chosen; check the accounts and the amount before you confirm.

The transfer is instant and free. You find it in Wallet → History → Transfers.

Rules:

Only the same asset can be transferred (for example, USDT to USDT). To exchange one asset for another, use Swap or place an order.
The main account can move funds to and from its sub-accounts. A sub-account can send funds to the main account, but cannot take funds from it. Funds cannot be moved directly from one sub-account to another: go through the main account.
Signed in as a sub-account, you see the main account only as a recipient: its balances are not shown, and the arrows button does not swap the direction while the main account is chosen. A transfer to the main account cannot be taken back by the sub-account, so check the amount before you confirm.
Nothing can be transferred from a frozen sub-account (it is marked frozen in the lists) until it is unfrozen; transfers to it are allowed.
You can transfer only the Available amount. Funds reserved by open orders or withdrawals cannot be moved.

To send funds to another person, use Send: How do I withdraw crypto?

Stay safe: Strade support never asks for your password, 2FA codes or seed phrase.

How do I move funds between my accounts?