swap iCloud accounts

What you consider primary and secondary is completely your choice. Accounts cannot be merged, so any purchases, subscriptions, photos, other content, etc. will always be tied to the Apple ID used to purchase, take the photo, etc. Generally, it's not good practice to have multiple Apple IDs.

See a thread switching from secondary authentication t… - Apple Community

Read If you sign out of iCloud on Mac - Apple Support

Note : your primary Apple ID is say example1@icloud.com and secondary Apple ID is example2@icloud.com it depends if the account is set with 2FA or once again created with secondary authentication not from this Mac , but from another Mac / iPhone / iPad , that can be signed in empty fields with further upcoming steps .

Second method is change Apple ID Change your Apple ID - Apple Support

Applicable for a single Mac in which other devices are not synced .

If other devices are synced this article has to be followed What to do after you change your Apple ID or password - Apple Support