I hope i am saying this is the right context, but step in can be when a person becomes involved in a situation/problem they weren't involved in before, and step up can mean that someone who is already involved improves themselves or their actions regarding the situation/problem.

Hi Ethan, step in is often used to mean "intervene" or "to help out by substituting/covering" You can use it in a situation where someone who's stronger/more powerful/has a higher position than you intervenes to help you deal with a problem. For example, "As an older sibling, you should step in if you see your younger sibling being bullied." or "When the main singer lost her voice, Natalie stepped in and took over." Step up would be to volunteer to take up a position of responsibility. For example, "In times of war, young men should step up and be ready to fight for the country." or "Step up and face the consequences of your actions." Hope this helps！（真巧！）

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