The phrase "in which" is a common way to introduce a subordinate clause and provide additional information about something. However, if you want to vary your writing and avoid using the same phrase repeatedly, there are several synonyms that you can use. Some possible options include "wherein," "whereby," "during which," "with which," "on which occasion," "at which time," and "within which." Each of these phrases has a slightly different connotation and usage, so it's important to choose the one that best fits the context and meaning of your sentence. Experiment with different synonyms to add variety and interest to your writing.