How to not end a sentence with a preposition?

I know this experience would be life changing and I could learn a lot from those with whom/what I'd be working.

Rahul’s answer is correct.  Note though, that it is not necessarily wrong to end a sentence with a preposition.  A preposition can be part of a phrasal verb.  Here’s a section in Grammarly Handbook about that.  http://www.grammarly.com/handbook/grammar/verbs/32/phrasal-verbs/  You can also search above your question for other questions and answers about ending a sentence with a preposition.

Another way to change your sentence is to use different words altogether.  What are the people called that you will be working with?  Are they coworkers, clients, patients, families, children, disabled people, or foreign exchange students?  Instead of “those I’d be working with,” you can use my future coworkers, the families seeking help, or the students.  The reader likely already knows that you would be “working with” these people.  So, change the focus to include those people in the sentence.