Non superusers
DotNetNuke supports lost password requests. To do this for non-superusers (such as host) just click the login link and then click retrieve password. Some sites may not support password retrieval (this is possible via web.config settings) you may see a reset password option.
Please note, superusers such as host cannot use this function as DotNetNuke will not send out their passwords (instead it sends a series of asterisks)
Superusers
There are a number of ways to reset superuser passwords, most of which work around creating a new account with a known password and copying over the password and salt values. The following 2 articles describe ways to do this
However if you have FTP/filesystem access then you can also recover the password via an aspx page.