How to change the wordpress user role inside database?

I want to know what changes should be made (or which meta_key and meta_value should be inserted) inside WordPress database in order to change the WordPress user role.

I already found out how to change it to administrator by inserting

`meta_key`:`wp_capabilities`, 
`meta_value`: `a:1:{s:13:"administrator";s:1:"1";}` 
and 
`meta_key`:`wp_user_level`, 
`meta_value`:`10`

But what I want to know is how to insert a user with different user role, or how to change an existing user role.

I already know WordPress has specific functions for it, but I want to do it via a custom php code.