On the login-page there is a "Forgot password" link, however this only works if the admin user is configured with an email address and an smtp-server is configured for Clerezza to send mails.
You can reset the password on the Clerezza shell (which is accessible without password on the console on which Clerezza is started) withe the following commands:
zz>import rdf.core.access.TcManager
import rdf.core.access.TcManager
zz>val tcm = $[TcManager]
tcm: org.apache.clerezza.rdf.core.access.TcManager = org.apache.clerezza.rdf.core.access.TcManager@6de35b46
zz>import platform.Constants
import platform.Constants
zz>val sysGraph = tcm.getMGraph(Constants.SYSTEM_GRAPH_URI)
sysGraph: org.apache.clerezza.rdf.core.access.LockableMGraph = org.apache.clerezza.rdf.core.access.LockableMGraphWrapper@39b7e5ee
zz>import rdf.core.impl._
import rdf.core.impl._
zz>import rdf.scala.utils.Preamble
import rdf.scala.utils.Preamble
zz>val p = new Preamble(sysGraph)
p: org.apache.clerezza.rdf.scala.utils.Preamble = org.apache.clerezza.rdf.scala.utils.Preamble@670583f6
zz>import p._
import p._
zz>import rdf.scala.utils._
import rdf.scala.utils._
zz>import rdf.ontologies.PLATFORM
import rdf.ontologies.PLATFORM
zz>val user: RichGraphNode = new PlainLiteralImpl("admin")/-PLATFORM.userName
user: org.apache.clerezza.rdf.scala.utils.RichGraphNode = org.apache.clerezza.rdf.jena.commons.JenaBNodeWrapper@957eb29c
zz>import rdf.ontologies.PERMISSION
import rdf.ontologies.PERMISSION
zz>user.deleteProperties(PERMISSION.passwordSha1)
zz>user.addPropertyValue(PERMISSION.passwordSha1, "d033e22ae348aeb5660fc2140aec35850c4da997")
The above will set the password of the user admin to "admin". Replace the value of PERMISSION.passwordSha1 to the sha1 sum of the the desired password, which you may compute on a typical unix shell with: echo -n password | sha1sum