How do i increase length of existed link title field in Drupal 8?
What you can do is this:


Create new link field
Update configs, like form display, views etc to use new field.
Migrate all the data to the new field, in an update hook. Example code could look like this (Assuming link field is on a node)

$nodes = \Drupal::entityTypeManager()-&gt;getStorage('node')-&gt;loadByProperties(['type' =&gt; 'node_type');
foreach ($nodes as $node) {
  $node-&gt;new_field = $node-&gt;old_field;
  $node-&gt;save();
}

Delete old field.
Done


Alternately you can update field field, manually update the SQL tables etc, but this is quite tricky to do properly. If you want to go down this road you should take a look at the entityDefinitionUpdateManager.