You will need to spin up a new VM with the SQL 2019 Installer in C:\SQLFullInstall then map this to the server you are upgrading

Backup Encryption Keys

Backup DB’s

Backup Settings

Run the SQL 2019 Installer and Run the Upgrade

*Save the CD Key used embedded as you will need this from the SQL Reporting Installer

Remove Reporting Services and Install the new version

Install SQL Reporting Installer with Key from the SQL Enterprise Installer

Open SQL Reporting Configuration and set the DB back to ReportingServer

Restore the Encryption Keys

Restore the Settings

Some Settings SMTP will be in the config file like SMTP Port

Old Server

C:\Program Files\Microsoft SQL Server\MSRS13.MSSQLSERVER\Reporting Services\ReportServer

New Server

C:\Program Files\Microsoft SQL Server Reporting Services\SSRS\ReportServer\rsreportserver.config

Remove the Old One

Logs

C:\Program Files\Microsoft SQL Server Reporting Services\SSRS\LogFiles