You can add more FLOW to an existing stake at any time.

1. Click Staking in the left-hand sidebar and open FLOW staking.
2. Click the FLOW account to show its stake accounts.
3. Find the stake account, then open the three dots (⋮) menu and choose Stake more tokens.
4. Enter the extra amount and click Continue.
5. Review and approve the transaction.
6. Click Close.

The new FLOW shows as Committed until the next epoch starts (up to about 7 days), then becomes Staked and starts earning.