Decompress as in take and make the file bigger/what it will be like when it hits RAM?
If so then you either need to find/make a compression tool or find some way to skip the decompression. If you have some tools to handle the decompression part for the former then they tend to come with either source code or documentation (or documentation is available somewhere) and you then get to reverse the operation.
Skipping the decompression is a bit of a cheat but a viable method. Two main approaches are you change the game's read command from the grab and decompress option to just copy directly, and the other is you add in the skip compression commands*.
*varies depending upon what the compression is but as files can be counted upon to have a unique section without a repeat you will usually have a feature of the compression to note that somehow. You put this "nothing to see here" command in every relevant location within the file and you have a "compressed" file that will run through the decompression feature just fine but have no need to figure out its full parameters. That is a wordy version though. For something like many forms of popular LZ family compression then that means every 8 or however many bytes where you have the flag for it you use the "no compression here" command. Huffman can have something similar but you might have to make a fake library of some form.
Beyond that you probably have the usual pointer issues. This is to say for systems that use the incbin style methods of putting everything in the binary with no file system (which is to say everything GBA and older and everything N64 and older if it did not come on a floppy disc or optical disc) then if you make it bigger you can't just punt everything that follows it forwards, and similarly if it is smaller you can't shuffle it backwards (though you can pad it out to match sizes). In some cases you can repoint the edited file to the end of the ROM and thus not have to repoint everything that follows (or otherwise tweak things to fit) but that sometimes only works for a handful of files before you run into limits (not sure what the limits of the N64 are off the top of my head).
For the sake of being relatively complete when you say edit I assume you are editing with a tile editor or something that keeps it in the original format and are not just converting to some common PC format and editing that instead. If you are then you will either want to stop that and fine something that edits the original files or make a tool to convert back, except you won't have any simple workarounds (though probably some better documentation because it is hardware and not a custom compressions scheme).