Select the "spanner" icon (near the top on the left hand side). From the options that pop up select "drill". Then select a size. If you don't fancy one of the sizes on offer, pick the dots at the bottom of the list & type the value you want in the box. Then click on the hole you want to change. You can do the rest just by clicking on them without having to select the spanner again.
A shortcut is to type "cha dri 0.1" to change it to 0.1".
vinodquilon said:
One more doubt.
Does it is possible to use multiple wire widths on the same PCB ?
Absolutely. It is normal to route the power with wider tracks. I use at least three sizes on a PCB - one size for power, one for normal routing and a narrow size for squeezing through tight spots.
Auto routing should follow the rules you set so should not create clearance errors, although I hardly ever use it so I am not sure if you can end up setting a contradictory set of rules e.g. one set for routing and another set for the DRC.