Hi,
AcyMailing 5 and AcyMailing 6 are not the same components, this is why they're no button update.
For the moment AcyMailing 6 is still under development, it has a lot of features but as much as AcyMailing 5.
I recommend you to install AcyMailing 6 on a test environment to see if version 6 fulfill your needs.
If so the procedure to update:
- Connect to your FTP and delete the folder acymailing in
/wp-content/plugins/
- Install in front end the last package of AcyMailing 6
- Go in AcyMailing and run the migration script to have all of your data