How can I continue using SQL Server 2025 Preview with full features after the 180-day trial ends?

How can I continue using SQL Server 2025 Preview with full features after the 180-day trial ends?

You simply can't. You also can not "upgrade" a CTP to a regular RTM edition, that's also not supported.

You have to backup your databases, uninstall the CTP and make a new installation of the RTM version.