composer update drupal/drupal does not work : Nothing to install or update
Finally, i could update with these modifications.

Step 1, optional in case your composer.jsoncontains this. If it is ok, apply step 2

add 

"require": {
        ---
        "drupal/core": "~8.5"
    },


remove

"replace": {
    "drupal/core": "^8.4"
},


Step 2

delete this folder :

vendor


delete this file :

composer.lock


run 

composer update --with-dependencies