Hi,
You'll find the documentation here:
https://docs.acymailing.com/
https://docs.acymailing.com/setup/migration
V7 is completely rewritten. When you install it it will be a new component in Joomla and v5 and v7 can co-exist on the same server. Please note that you can attach your license (nneded for the automatic sending) to only one of them at a time.
So you safely install v6 to get familiar with it. Just leave your license attached to v5. You can even choose to migrate your data at a later time.
Both versions use their own set of database tables. Once you're completely migrated and have no need for v5 anymore you can safely uninstall it and delete it's database tables.
When migrating global statistics will be available in v7, but the detailed statistics will not be migrated. If you use Smart Newsletters you need to recreate them in v7 as Automated campaigns.
Please check if you use any special plugins in v5 and check if these functions are already present in v7. In v7 they are referred to as add-ons.
Worth noting: When switching to v7 you need to delete the v5 subscription modules and replace them with v7 ones. If you don't do that new subscribers will still be added to v5 instead of v7.