Welcome to the forums,
Welp, I assume you've already
compared the features you will be giving up, weighed what you can afford, talked to sales to get a quote, etc, etc.
Anyways, you essentially install the Open Source edition over the top of your NE install to keep your data and convert to the FOSS edition.
What version are you running? You technically don't have too - but it's best if you use the same zimbra version for the switch to keep it simple.
In addition to any backups you run via zmbackup in NE; please make a backup of your /opt/zimbra (and any directories linked elsewhere)
Open Source Edition Backup Procedure - Zimbra :: Wiki
about as simple as you can go is:
su zimbra
zmcontrol stop
switch to root
ps aux | grep zimbra (kill -9 any leftover)
mkdir /backup
rsync -avHK /opt/zimbra/ /backup/zimbra
Of course you can always go back should you decide so later:
Moving from Open Source to Network Edition ZCS - Zimbra :: Wiki
NE 5.0:
/products/downloads.html
NE 5.0 Trial:
/products/download_network.html
FOSS 5.0 :
/community/downloads.html
FOSS prior versions:
/community/downloads_previous.html