UDAB's Super-Brief guide to retuning DAB Radios:

DAB radios need to be scanned to find new stations. Before you begin, be sure to put the radio’s telescopic aerial right up and vertical. If possible, move the radio upstairs and near a window. If the radio has an auto-tune / auto-scan button, just hold it down for at least three seconds to get the ‘full scan’ option. If there’s no obvious auto-tune or auto-scan button, it’ll be in the menus.
Once scanned, new stations should be in your station list and you can then save your favourites to presets, often like an FM car radio by holding down the preset button while tuned to the station.
The ‘Trial London’ multiplex broadcasts to inner London, out as far as the A406 North Circular & A205 South Circular roads; maybe to a little bit further in cars with good systems.
For manual tuning, it is found at Block 9A (frequency 202.928MHz).