You can remove a downtime in a monitor by manually deleting it from the web client. Navigate to Home > Outages > click on the hamburger icon at the far right for an outage > Delete the outage.
You can also delete an outage from the details page of the monitor. Click on the outages tab > click on the hamburger icon on the far right > delete the outage for that monitor.
This will delete the outage and won't be reflected in the SLA report. Hope this helps.
Product Manager, Site24x7