Yes, you can. If you copy your .apk file so that it has a .zip extension, you can unzip it like any regular zip archive. From there, you can get your help and res folders, as well as your AndroidManifest.xml.
If you have written extensive Java code, you will need to decompile the classes.dex file back into Java. Here is a good SO question that covers it:
decompile DEX into Java source code
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