You are "encrypting" (really hashing) your passwords using MD5. Because it is being hashed you can't/don't decrypt it, you merely compare your hashes to decide if the user is authenticated or not.
If you are wanting to decrypt the password as a easy way to remedy "i forgot my password" you are better off implementing that functionality as part of your App in case your system is compromised and all your users passwords are uploaded to the internets.
But to answer the question specifically, you need to use an encryption algorithm (ex: blowfish) and not a hashing algorithm. Take a look at blowfish for example.
Google[
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GitHub - b1thunt3r/blowfish-csharp: Blowfish encrytion library for C#[
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Blowfish C# and C++ Source Code - Defuse Security[
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Koders Code Search: Blowfish.cs - C# - GPL[
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