1. We only found the Update Guide, not Upgrade Guide, are these two guides the same thing now?
Yes, it is same now. In https://docs.pega.com/update, you get a form which contains field "What is your current Pega release", select appropriate option. Select appropriate answers to other fields. This should generate a required document to update Pega.
2. According to the Update Guide content, some chapters are not applicable to our version, so we just need to ignore these processes？or do some other oprations replace?
Please follow relevant sections and process based on source and target Pega version. All required steps are mentioned in the document.