I am creating a new component with several parts in Altium Designer. When I place the component in a schematic page, each part get a unique letter like (IC10A, IC10B, IC10C ... etc). How to change these letters to numbers, like (IC101, IC102, IC103 ...etc)
You can do this via Tools -> Board Level Annotate.
You will need to select $ComponentPrefix$GlobalIndex in the "Annotate Options" screen.
However, this is uncommon for Altium and "Copy room format" might behave erratically, so I wouldn't recommend it.
You can always set custom designators for your physical components. To do this, you will need to compile your schematic (c->r) and in the bottom of your schematic you will then see the name of your compiled sheet, eg. U_IO1, U_IO2, ... Switch to one specific sheet and then double click the designator. This will give you the "Physical Component Properties" screen where you can assign a designator manually as well. Again, not really recommended.