The short, not full and not acurate list is:
1. Find out the path of upgrade in documentation. If your version too old it could be multistaging upgrade. For example: 7.0.x > 7.4.x > 23.2
2. Create new deployment on new hardwade.
3. Test your customisation modules and models. Make shure it is compatible with new version. Remove all uncompatible.
4. Copy your database and contentstore from old to new deployment.
4. Start clean alfresco and monitor the upgrade.
5. Reindex search engine.
Documentation here:
https://docs.alfresco.com/content-services/latest/upgrade/
