Upgrading Kerio Connect to 64 bit is not difficult. Of course you do need to be running a 64 bit version of your operating system. On Linux or Mac, you simply proceed as you would for a version upgrade: download the new version and install it. See Kerio's Knowledgebase article Upgrading Kerio Connect. You should read that in addition to this.
On Windows 64 bit there is a little more to it. Your 32 bit Kerio would have been installed in C:\Program Files (x86)\Kerio\MailServer\ and the new 64 bit will install to C:\Program Files\Kerio\MailServer\. Therefore, certain files must be moved from C:\Program Files (x86)\Kerio\MailServer\. This is very much like transferring to a different operating system, so Kerio's Transferring an installation of Kerio Connect to another server or Operating System does cover the concepts, but I'll spell them out here because there are some subtle points you could miss.
Configuration files and SSL certs
The *.cfg files in C:\Program Files (x86)\Kerio\MailServer\ must be moved to C:\Program Files\Kerio\MailServer\ AFTER shutting down your newly installed 64 bit version. The certification files in C:\Program Files (x86)\Kerio\MailServer\sslcert must also be moved.
There is a cluster.cfg file also but I haven't seen that used yet. It's empty wherever I have seen it.
That's easy enough, but there can be a complication with the Store directory where all your mail is actually stored.
The Store Directory
When you orginally installed Kerio, you could have put the Store directory anywhere. You might have put it on its own drive at M:, for example. If you did, there's nothing more to do: you are ready to start up the 64 bit and trash the old 32 bit version.
However, the default location is C:\Program Files (x86)\Kerio\MailServer\Store\. You COULD just leave it there, but that could cause confusion later - someone might remove the apparently unused C:\Program Files (x86)\Kerio\MailServer\ directory. It woould be better to move it and that will require changing the mailserver.cfg file you copied to C:\Program Files\Kerio\MailServer\ above.
There are two ways to do that. One is to start up the 64 bit and change the Store directory location in the Admin panel under Configuration -> Advanced Options -> Store Directory tab
The other is to edit the mailserver.cfg and locate these entries: