1) you can pay back any amount up to the full amount. deposit with the custodian.
Some custodians are kind enough to add this option to online contribution page.
Otherwise, make sure custodian understands your intent.
An IRA payback would be simpler than a 401K payback.
BUT, you can pay back the 401K COVID withdrawal into an IRA instead if you want.
2) you must amend your return (Form 1040-X) to get your refund.
