Backup your 5.3 Avalanche instance by following the instructions found here:
Save your backup file in a safe place.
You will need to prepare your new server for install by installing Microsoft SQL. Recommendations, walk through and supported versions can be found at the below link.
Once you have completed the SQL install (as described in the Best Known Method install Avalanche 6.2 article),
Complete the recommended steps described below for upgrading from 5.3 to 6.x.
After which return here: Best Known Method Install Avalanche 6.2 and complete the upgrade.
For upgrading from 5.3 to a 6.x installation it is recommended to do this from your new server and pulling the old install from the old server.
You'll first need to make sure that no firewall settings will be blocking the default port of 5432 for the Postgres installation. This will also include adding a line to C:\Program Files (x86)\Wavelink\AvalancheMC\db\data\pg_hba.conf that allows traffic to the database from your new server. The line will look like what is highlighted below (host all all 10.1.1.1/24 md5). Note: this is only an example that would range the last digit of the IP address listed to the 24th octet. The preceding decimals would need to match that of your domains IP range.
You will now need to restart the 5.3 server for the above changes to take effect.
Open and run the installer. Select your language and click OK.
Go over the Pre-Install checklist and click Next.
Accept the licensing agreement and click Next.
Define your install location and click Next.
Select Upgrade and typically you will select Postgres as the database type. Click Next.
Select your component install options and click Next.
Define the path from your old installation, the username, password, port and database name. Note: Typically the port and database name will be defaulted.
If you do not know the password for your 5.3 Postgres database you can follow this doc to retrieve that information:
The next page refers to the new database information on your 6.x server. Fill in the Hostname, Username, Password and Port if you are not using the default of 1433. Click Next.
Click Yes to each of the next two screens.
Define your Central Web File port, or leave at default and click Next.
The next screen is your chance to review everything, go back and make changes or click Next to continue.
UPDATE: This has been added in 6.2.2:
Please click the link and review the article that is displayed from the help before acknowledging. This step is a requirement for any backup and restore of Avalanche going forward. Acknowledge and click Next.
Allow the install to complete.
Finally, make a copy of your avapackages folder from C:\Program Files (x86)\Wavelink\AvalancheMC and paste and merge that folder with the avapackages folder on your 6.x system located at C:\Program Files\Wavelink\Avalanche.
You can now launch Avalanche.