The first appeal, filed from abroad
The first challenge to an assessment is an appeal to the Commissioner (Appeals) under Section 246A. You file it in Form 35, online through the e-filing portal, so being abroad is no obstacle, within 30 days of being served the assessment order or the demand, and with a modest appeal fee. The 30-day limit is strict, though the Commissioner can condone a delay for a genuine reason, so it is best filed promptly.
The appeal is heard faceless, under the current faceless appeal scheme, so there is no officer to appear before in person; the case is processed online, and if you ask for a personal hearing, it is held by video. So the whole first appeal, from filing to hearing to order, can be run from wherever you are. Which appellate authority hears it depends on size: smaller cases go to a Joint Commissioner (Appeals) tier, and larger ones to the Commissioner (Appeals). The procedure and the authority's powers, including to confirm, reduce or even increase the assessment, are set out in the following sections of the law.
The stay link, and the further appeal
The first appeal works hand in hand with the stay of demand. Filing the appeal is what lets you apply to stay recovery of the disputed demand, usually on paying part of it, so that your Indian assets are not pursued while the appeal is decided. So the appeal and the stay are best filed together, not one after the other, to protect you from the moment the demand lands.
If the first appeal does not go your way, it is not the end. A further appeal lies to the Income-tax Appellate Tribunal under Section 253, filed in its own form within 60 days of the first-appeal order. The Tribunal is an independent body that can confirm, alter or set aside the assessment, and it is often where NRI disputes on treaty and residency points are genuinely won. A practising CA files the first appeal in time, runs it faceless on your behalf, keeps the stay in place, and takes the matter to the Tribunal if needed.