Unfortunately there is nothing you can do but continue to wait. Just about when you received the letter the IRS shut down all their processing centers because of the Covid-19 pandemic, and stopped all manual processing of tax returns. So they haven't done anything since you got the letter. They just recently started to reopen the processing centers, but they have limited staff and a huge backlog, so it may be quite a while longer before they complete the review of your tax return.
