Changing a theme in in both XOOPS 2.0 and XOOPS 2.2 is to be made in exact the same way.

Login with your administrator user and go to the admin control panel, from there enter Preferences and there select "General Settings".

Once there you will notice as the 8th field from the top is called "Default Theme" and is presented as a dropdown menu reflecting the themes you've uploaded to your server. Select the desired theme and then scroll a bit down to find the field called "Selectable themes", like its description says..."Choose themes that users can select as the default theme"...this field is for setting the loading theme for the user side of your XOOPS site, so it will have to be set acording the settings of the previous field... if you want to let your users chose from different themes all you have to do is select more than one theme, and than make the block named "Themes" vissible for the groups you wish to grant access to this option.

Good Luck