As you progress you'll unlock tokens - e.g. Base Tokens for completing bases, Crime Tokens for stopping crimes like robberies, etc.
You can use these tokens to upgrade your equipment and gadgets, you just need to make 15 of these upgrades.
You should be able to do this pretty easily and you'll get the tokens through natural progression (though the higher levels can take a bit of grinding - the first 15 are pretty straightforward).