I suggest you follow the explanation in the document @johnbishop sent you https://www.rxfiles.ca/rxfiles/uploads/documents/deprescribing-ppi-patient-tool.pdf
So, I also suggest to taper Nexium off slowly and then you can take pepcid to manage any symptoms you experience in between. Since you've been taking 40 mg, you can also instead take 20 mg pills for a bit, or every other day. If you've been taking Nexium for years, what's a few more weeks... in order to reduce the acid rebound.