Altium: How to change several components at once in order to save time and effort?
Learn to love "Find Similar Objects". Right-click on the object, change 'Value' from 'Any' to 'Same' and it should find all the 1pF capacitors. 

It will highlight them when you click apply, and if that's right (and it should be, unless there are more than one type of 1pF capacitor, in which case you should apply some distinguishing feature) then click 'okay' to bring up the SCH inspector dialog. 

You can then change the value to whatever you like, as well as change many other characteristics. The changes are applied immediately, not through the usual validate and execute.