“上诉不加刑”是中国刑法中的概念，翻译时需先了解其内涵。这一原则源自刑事诉讼法（Criminal Procedure Law）第二百二十六条：

第二百二十六条 第二审人民法院审理被告人或者他的法定代理人、辩护人、近亲属上诉的案件，不得加重被告人的刑罚。第二审人民法院发回原审人民法院重新审判的案件，除有新的犯罪事实，人民检察院补充起诉的以外，原审人民法院也不得加重被告人的刑罚。

人民检察院提出抗诉或者自诉人提出上诉的，不受前款规定的限制。

官方搜索的对应译文如下：

Article 226 When trying an appeal case filed by the defendant or his/her statutory representative, defender or close relative, a people’s court of second instance shall not aggravate the punishments on the defendant. Where a case is remanded to the original people’s court for new trial by the people’s court of second instance, unless there are new facts of the crime and the people’s procuratorate has initiated supplementary prosecution, the original people’s court shall not aggravate the punishments on the defendant.

The restriction laid down in the preceding paragraph shall not apply to cases protested by a People's Procuratorate or cases appealed by private prosecutors.

由上可知，“上诉不加刑”其实就是not aggravate the punishments on the defendant in an appeal case。但在题主问题的语境““上诉不加刑”原则”中，若如上所述将意思全部翻译出来，未免不够简洁，因此也可以借鉴美国第五修正案中提及“Imposition of Multiple Punishments for the Same Offense”时进行的描述：

Appeal resulted in no further trial or other proceedings to which a defendant might be subjected, only the imposition of a new sentence. An increase in a sentence would not constitute multiple punishment, the Court continued, inasmuch as it would be within the allowable sentence and the defendant could have no legitimate expectation of finality in the sentence as first given because the statutory scheme alerted him to the possibility of increase. Similarly upheld as within the allowable range of punishment contemplated by the legislature was a remedy for invalid multiple punishments under consecutive

Sentence Increases.—The Double Jeopardy Clause protects against imposition of multiple punishment for the same offense. 

因此，也可借鉴上文，将这一原则译为“the principle of appeal resulting in no sentence increases/additional punishment”。