The current release is 6.4.0.3-4.1.0.1_45063. The TAC site will generally be more current than the automatic updates as it can take 2 - 4 months before a version released to TAC is posted to the automatic upgrade servers.
You can jump straight to the current release using your own TFTP server. The -135 and -105 use different images so you will need to download a copy of each file and have them available.
Depending on the version you are running you may have this view under maintenance. This allows you to specify the image locations of a mixed cluster (in this case it is -225 and -115).
If you do not have this option then you would want to split the network so you either have two networks -- one for -135 and one for -105s the upgrade each network then re-join them. You could also just shut down the -105s, upgrade the -135s, then shut down the -135s and bring up the -105s and upgrade then bring up the -135s.
NOTE: The Automatic upgrade will upgrade all the IAPs for you -- both the 105s ad -135s will get their code from the cloud.
Are your IAPs being managed by AirWave?