There's no scheduling feature built-in in updown at the moment (see: https://updown.uservoice.com/forums/177972-general/suggestions/32393419-allow-maintance-window-for-planned-it-jobs) but you can automate it on your side using the API to disable and enable the check at the time you want (for example in some maintenance script or at a fixed time in a cron task)
For example using the curl command you can do it this way:
curl -X PUT -d 'enabled=false' https://updown.io/api/checks/:token?api-key=:apikey
You can find your API key on the API page and your check "token" is a 4 letter identifier you can find from the URL of your status pages (for example https://updown.io/ngg8 → the token is "ngg8") or from the API using the
Alternatively if it's not too often you can simply use the interface to manually disable the check before and re-enable it after the maintenance window, using the toggle on the left of check edit form.
While the monitoring is disabled, no requests will be performed so updown will not detect any downtime or send any alert. The status of your website (in the UI / status page) will simply stay the same as it was on the last check.