To Import a BibChip Crossreference File, go to Participants > Actions > Setup Chip Auto-Assignment and upload a CSV spreadsheet of Bib and Chip. You should always upload a full crossreference of all expected bib numbers. As Participants are registered, they will automatically be assigned the Chip that is affiliated with their Bib using the file you have provided.
If you have a single Chip Crossreference loaded (for instance if you were substituting a single bib number with another bib number) you must also provide a one-to-one BibChip crossreference for the other bib numbers that did not change. You can use the "Add One-To-One Chip Assignments" function to accomplish this.