can you change apple watch sport band to a leather band?

Yes, if you meant buy a new leather band and change it.

Hi

Any 38mm or 42mm Apple Watch band will fit an Apple Watch of the same case size.

Non-Sport bands have different lengths, so check your wrist circumference and refer to the Apple Watch Sizing Guide (which compares all Apple bands) before buying separate bands:

http://www.apple.com/shop/watch/sizing-guide

The Apple Watch Sport also has an anodized aluminum case, whereas non-Sport bands have different metals in their adapters and/or links (this does not affect compatibility).

Information about how to change bands over:

Change your Apple Watch band - Apple Support