Use any hex editor apps that are available for Android and change the value of HEX Dword to any amount you want.
Crystals HEX 34 42 93 a1 Dword
Gold HEX 5a 4a ed a0 Dword
If the codes doesn't work, it is possible that your android version is not compatible with the code I provided above.
Fortunately you can do it manually! To do it manually, just use search function in your hex editor app, enter your current Gold and Crystals value in MARVEL Future Fight and search for it (choose DWORD) .
If you see a small list of results, example 10. Just edit the value of them one by one until you can find the real HEX!
If you see a large list of result, you need to change the value of your current Gold and Crystals value in MARVEL Future Fight by either using/consuming or adding it. Go back to the list and find your new current Gold and Crystals value in MARVEL Future Fight to the large list and you can now edit the value any amount you want.