How to update wp_capabilities?

I want to assign another role to a user.

I understand the data is stored in the usermeta table in the metakey: wp_capabilities.

The data is serialized, such as:
a:1:{s:13:”administrator”;b:1;}

The method I used to update is:
update_user_meta($userid, $metakey, $value);

I have used maybe_serialized() to serialize the $value of one of these:
array(‘contributor’,’seller’) and
array(‘contributor’=>1, ‘seller’=>1) and
array([contributor]=>1, [seller]=>1)
without any success.

The user wp_capabilities is updated and serialized, but it seems storing the wrong value.

If I use this tool to unserialize
https://blog.tanist.co.uk/files/unserialize/
it won’t show the correct array back.

I have searched everywhere but no page can explain how to do it correctly.

And I can’t find the answer on wordpress codex either.

Does anyone how to update wp_capabilities of a user, or any other method to assign another role to a user?