Buy a license for SQL Server Standard Edition. I don't know if you can buy one for SQL 2016 specifically, or they only sell licenses for SQL 2019. But if so, the downgrade should not be a problem. (Then again, why you go with a version that runs out of mainstream support later this year?)
Once you have the installation media, run Setup and select Edition Upgrade to upgrade from Evaluation Edition to Standard Edition.
Make sure that you still have the same version number after the installation. If it is lower, download service pack 2 and reapply it. (I don't do these operations very often, so I don't know exactly how it works.)
You should also download and install the most recent Cumulative Update for SQL 2016 SP2 which is CU17 if I remember correctly.
Before running the installation, you may want to run this in your database(s):
SELECT * FROM sys.dm_db_persisted_sku_features
So that you have used thing that will not work in Standard Edition.
Note that there is no "activation" per se.