Hi 

Would like to check which REST API can be used to update rate limit policy and then publishing the changes to gateway? Thank you

This is APIC 10.0.5 REST API doc. 

https://www.ibm.com/docs/en/api-connect/10.0.5.x_lts?topic=information-api-connect-rest-apis" href="https://www.ibm.com/docs/en/api-connect/10.0.5.x_lts?topic=information-api-connect-rest-apis" rel="noreferrer noopener" target="_blank" class="fui-Link ___1rxvrpe f2hkw1w f3rmtva f1ewtqcl fyind8e f1k6fduh f1w7gpdv fk6fouc fjoy568 figsok6 f1hu3pq6 f11qmguv f19f4twv f1tyq0we f1g0x7ka fhxju0i f1qch9an f1cnd47f fqv5qza f1vmzxwi f1o700av f13mvf36 f1cmlufx f9n3di6 f1ids18y f1tx3yz7 f1deo86v f1eh06m1 f1iescvh fhgqx19 f1olyrje f1p93eir f1nev41a f1h8hb77 f1lqvz6u f10aw75t fsle3fq f17ae5zn" title="https://www.ibm.com/docs/en/api-connect/10.0.5.x_lts?topic=information-api-connect-rest-apis">https://www.ibm.com/docs/en/api-connect/10.0.5.x_lts?topic=information-api-connect-rest-apis

The plan rate limit information is in the product document. If you want to update that then I think you need to publish an update to the product to contain new rate limit information - including migrating users from the old product to the new one.
I do not think there is a way to update the rate limit information for a published product directly, only by publishing a new version of that product.