Upgrading Drupal

just got a site (www.yanaihome.com) which is Drupal 7.94, want to upgrade to the newest version, how to approach that?

some link or documentation will be great appreciated.

can it be done straight to 9.5?

Thanks,

Mike

Im trying to migrate my drupal 7 site to 9 but i received this error:

For a project I chose to use the Olivero theme, which I like very much.
The problem is that this theme is in the Core package and this involves on the one hand, backing up the theme every frequent update so as not to lose customizations, and on the other hand "missing" any updates to the theme itself.
I tried to investigate and as far as I could find it is almost impossible or very complex to make a sub-theme out of it.

Is there a smarter solution?

Many thanks

The website encountered an unexpected error. Please try again later.
TypeError: Illegal offset type in Drupal\migrate_drupal\MigrationState->buildDiscoveredDestinationsBySource() (line 349 of core/modules/migrate_drupal/src/MigrationState.php).

Why?

I would like to migrate a Drupal 7 site to a Drupal 9 site. I am an amateur user.
I have created an empty Drupal 9 site on a local server. Until then everything was going ok.
Now I have imported both the old and the new database in phpMyAdmin into localhost.
When accessing the source database I always get the error: SQLSTATE[HY000] [1045] Access denied for user...
The source database is the original one in Drupal 7, right? I took the name and password data from the old database. (There is an underscore in this name but that can't be a problem I guess)

We are trying to slowly upgrade our system and are wondering if anyone has used pgsql 12 with drupal 8. Eventually, we want to upgrade to drupal 9 and pgsql 14 but in the mean time if anyone knows if this works, please let me know. 

Thanks