This link on the ARRL website breaks it down clearly. Don’t let your renewal go too long — if it’s been expired for more than two years, it’s not eligible for renewal or reinstatement.
“Amateur applications can be filed manually using paper forms or electronically via the FCC website. Filing interactively is easy and the results are immediate.
To renew or update a license, amateurs may file manually using FCC Form 605 or file interactively on the FCC ULS website using their FCC-issued Federal Registration Number (FRN) and password. FCC permits administrative updates and call sign changes at any time. License renewals are allowed at 90 days or less before a license will expire, and when the license has expired but is still within the two-year grace period for renewal. Licenses that have been expired for more than two years are not eligible for renewal or reinstatement.”