You can set your website to "Site Maintenance" mode by using the software Site Maintenance Redirect App, available in the UWB Full and Lite versions.  No .htaccess file editing or coding necessary!

Go to Configure Site > Site Maintenance to configure it.

Here, you can set it On/Off.  You can set which page is the Site Maintenance page to which to redirect website visitors when Site Maintenance Status is On.  Alternatively, you can not set a redirect page and use the HTML Editor Box here to create a custom message to display on the page during Site Maintenance.  You can also choose which Admin(s), and list which IP Address, have access to the website during Site Maintenance.  Click the "Save" button at the bottom of the page to save your changes.

You can opt certain pages of your website out of Site Maintenance from the List Pages > Add / Edit pages.

At the top of the Add/Edit Page there is a checbox labelled "Show page when Site Maintenance is On".  Checkmark this for the page if you want the page to still be accessible as usual during Site Maintenance.