GST rules amendment extends timelines and revises registration, UIN, input tax credit declarations and payment mechanisms. Amendments modify procedural timelines, registration requirements, electronic declarations for availing input tax credit, payment channels and form content. Rule 3 extends a statutory period; rule 17 expands UIN application bases to include Ministry of External Affairs recommendations; rule 40 requires electronic declaration in FORM GST ITC-01 and allows Commissioner extensions; rule 87 prescribes challan validity and permits specific electronic and international payment routes; FORM GST REG-13 and TRAN-1 are revised for UIN applications and transactional disclosures.