How to set route table as non-main?

The main route table is the default route table that every VPC has, and it's automatically associated with every subnet that you create, which doesn't have an explicit association with any other route table.

You cannot change a main route table to a custom one directly, or vice versa. However, what you can do is create a new custom route table, configure it as desired, and then change the association of your subnets from the main route table to the new custom route table.