It was nothing serious能不能改成It was not serious.

一般来说可以。

It was nothing serious 意思是 “这没什么要紧的 ” 或 “无关紧要，不值得重视”。

It was not serious 可以翻为 “不严重”。

用哪个要看文章的语境和语气咯。

例：

1. Jay and I had a friendly disagreement. It was nothing serious though.

2. Mary had a mishap at work, but it was not serious.

例(1) 用 it was not serious 不适当。

例(2) 可用 It was nothing serious