how do i restore purchases after updating ipad?

Purchases for what - Apps, Apple Services, media, something else? Updating an iPad rarely requires any purchases to be restored - unless you have completely wiped/reset the device.

For Apps, media and services purchased from Apple, you simply need to be signed-in to iCloud and App Store using the same AppleID from which the purchase was made - as all purchases are forever tied the associated AppleID account.