The easiest method is to query and select objects (in this case, your vias) on the PCB using the PCB Filter tab, then use the PCB Inspector tab to modify the parameters about those selected vias. On the PCB filters tab, enter "IsVia" in the query field and make sure that the "select objects passing filter" and "deselect objects not passing filter" check boxes are enabled before applying the filter. (You can configure the mask/zoom options as well if you want.) At this point, all of your vias should be selected, and you can open the PCB Inspector tab to make changes to all of those vias simultaneously. At the bottom of the PCB Inspector, it should display the number of objects currently selected by the filter query. If you don't see any parameters listed, make sure the object type at the top of the PCB Inspector tab is set to "all object types" or at least vias in this case. As you make changes to the parameters listed in the inspector, those changes will be immediately applied to all the objects that are currently selected (and match the type of object defined in the PCB inspector object type). So you can change all the hole diameters, pad sizes, etc with a single operation. The PCB Inspector is a very useful tool, so it's important to get comfortable using it.
FYI, you can also easily select all vias using the "find similar objects" option from the top edit menu. You can limit your selection to vias with a particular hole size or other parameter. This can also be done through the filter tab as well using query language like "IsVia AND HoleSize='12'" etc, but requires familiarity with the query language and variables.
Good luck.