It would help if we knew exactly what you tried and how it failed. All you do is add a formula field with SUBSTITUTE({field}, ";", ","), then convert the formula into a multi select.

Is that what you tried?

This is what I tried, but it says : Sorry, there was a problem saving this field. Can’t save field because it causes a circular reference.

Is the problem the name of my column ?

You should have two fields you’re dealing with. The original field with the semicolons in it ({field}), and the formula field for the substitute ({formula}). Once you’ve converted the formula into a multiselect field you can delete the original.

I’m sorry, I’m not sure I totally understand.

Here’s a preview of what I’ve been doing so far. As you can see, it changes all my data to another column I have.

As @Kamille_Parks mentioned above, you need to create a brand new formula field, not customize your existing field.

OH GOD, IT WORKS! Thank you so much for both your help!