How do I know when there are governance proposals in the voting period?

You can periodically check the Governance sections on Keplr Dashboard

How do I vote on a Governance proposal?

Next to an active proposal within Keplr Dashboard, click on "Vote," select your decision and then click on "Confirm." Keplr wallet will request you pay for the processing fee and then click "Approve."

Can my validator's vote override mine?

It is possible, but it depends on voting power (percentage calculated on the basis of the tokens you've staked). You can also overwrite the vote of your validator in proportion to your voting power.

I changed my mind - can I change my vote?

You can change your vote at any time as long as the proposal is in voting period.