There are two generic approaches to perform the upgrade.
a. In Place Upgrade.
In this approach upgrade of both rules and data schema / single schema(if the deployment is not split schema) happens during the downtime.
b .Out of Place Upgrade.
In this approach Rules schema and data schema shall be upgraded separately. In this approach only data schema upgrade happens during downtime. If the existing deployment is single schema, single to split schema upgrade shall be performed to minimize the downtime. Please refer 8.5 upgrade guide.
3. Redeploying the application with the 8.5 war/ear deployable files. Please refer 8.5 upgrade guide for reference.
4. There are common UI issues which may arise post upgrade. please refer following community link for most common UI issues.