It can be a requirement for designs to have vias covered in solder mask - this helps prevent shorts, improved solderability and minimizes the chance of electrical shorts if the board comes into contact with other objects. However the opposite is often true for prototypes - the vias can be left open to make probing and debugging that much easier. It is easy to switch all vias between tented and open with just a short sequence of commands.
Each via can be controlled individually by bringing up the property dialog (right click > Properties).
If you have many vias this can be tedious and a different technique can be used. Apply the via filter (see image below) and then select all objects (CTRL+A).
Next select View | Object Inspector if not already active.
From the PCB Inspector panel the tenting option can be applied to all selected vias.
Don't try to the the View | Filter command to select the vias. This may appear to select all vias but the inspector will only operate on the one via.