In the product source, under ZimbraServer/docs, there are TXT files that list and describe the SOAP API commands. The main file is soap.txt but there are other soap*.txt files for specific areas of the API.
For account modification commands like you mention, you'll want to look at soap-admin.txt since those are admin commands.
If you don't have the product source, checkout this page on how to access the Zimbra public p4 server. Based on version of Zimbra you have (for example, ZCS 6.0.5 source is found under GNR605), you can sync the ZimbraServer/docs directory to get the TXT files.
Building Zimbra using Perforce - Zimbra :: Wiki
And for what it's worth, checkout this thread. It has a simple outline of how to change passwords for an account using SOAP. Might help get you started calling the SOAP API.
http://www.zimbra.com/forums/develop...-examples.html