Upgrading laravel, fixing lock file not up to date errors

I'm starting a big upgrade from Laravel 5.8 to 6x firstly and the composer install gives me warning The lock file is not up to date

So then i did composer update --lock and getting a bunch of illuminate require errors:

Problem 1
- illuminate/filesystem[v7.0.0, ..., 7.x-dev] require symfony/finder ^5.0 -> found symfony/finder[v5.0.0-BETA1, ..., 5.x-dev] but these were not loaded, likely because it conflicts with another require.
- illuminate/filesystem[v8.18.1, ..., 8.x-dev] require symfony/finder ^5.1.4 -> found symfony/finder[v5.1.4, ..., 5.x-dev] but these were not loaded, likely because it conflicts with another require.
- illuminate/filesystem[v8.0.0, ..., v8.18.0] require symfony/finder ^5.1 -> found symfony/finder[v5.1.0-BETA1, ..., 5.x-dev] but these were not loaded, likely because it conflicts with another require.

Is there a composer way to fix this or do I have to manually update the lock ? delete them ?

In require-dev of composer lock , i can see

 "illuminate/filesystem": "^5.5",

require has:

"illuminate/filesystem": "^5.1",

There are multiple references to symfony/finder in the lock file - not sure how to process this?

thx