How to change a primary key for constraint Auto_increment

I've created the following table

create table if not exists cursos(
nome varchar (30) not null unique,
descicao text,
carga int unsigned,
totaulas int,
ano year default '2018'

)default charset = utf8;

After adding a PK

alter table cursos
add primary key (idCurso);
alter table cursos add column idCurso varchar(10) after nome;

My difficulty how to make it now auto_increment

First attempt:

 alter table cursos add constraint idCursos varchar(10)  auto_increment;

help!