A domain name bought with Sarbacane is valid for one year. To keep using it, renew it starting five days before its expiration date. Renewal costs €25 excl. tax (one domain credit). Allow two minutes.
You are the account owner or an administrator.
You have a domain credit available. Otherwise, you are redirected to payment during the renewal.
You are within the five days before the expiration date, shown in the Expires on column of the Domains page.
In Sarbacane Campaigns, in the left sidebar, click the gear icon (Settings), then in the left menu, click Domains.

If the domain to renew is your default domain, temporarily choose another domain or Not customized (default) at the top right, in the Default domain list. Otherwise, the renewal cannot go through.

At the end of the domain's row, click the ••• menu, then Renew.

If you have no domain credit available, follow the payment page.
Once the domain is Activated again, set it as the default again in the Default domain list.
The domain stays in the Awaiting validation... state for a few minutes, then returns to Activated, with a new expiration date one year later.
Note: before the expiration date, the account owner receives several reminder emails, and a banner appears in the interface.
Check that you are within the five days before expiration and that the domain is no longer set as the default (step 2).
The Domains page shows “Expired for N days”. The time allowed to renew depends on the extension. For a .fr, for example, you have 28 days to renew it with a new credit. After that, you must wait 80 days, or ask your account manager to restore the domain (paid service; they will tell you the cost). Deadlines by extension are listed on the Gandi website.