You’d have to duplicate the policy and set the severity to your desired value. Please note that anomaly based detections (policy severities) can’t be overwritten. Give it a try...

You are right Eddy

Would also maybe suggest updating policy to filter out know items you don't care about vs lowering the overall alert.

Example: Alert new external connection
Don't just disable or lower this alert, update the alert so that you exclude edge servers or edge apps like proxies or gateways

This way you can clean up your alerts but still get alerted when you need it, if it happens else where then known areas