It depends. Simplest is to make additional tank(s) and add some fuel pump/valves/pipes. Sure, it costs minus part of payload. In some cases, of very good opti

It depends. Simplest is to make additional tank(s) and add some fuel pump/valves/pipes. Sure, it costs minus part of payload. In some cases, of very good optimized designs, enough just to load less payload then mtow, to extend range significantly. For example Mooney m-20 with 3 persons fly farther than with 4 persons. Hard way, to install engine(s)/prop(s) with better fuel economy, and to optimize aerodynamics with better foil, larger wingspan, v-tail, winglets, laminarization, etc. And it is not guaranteed to achieve extending range, by just do right enhancements. For example, with An-2 modern upgrades, range not extended, but becomes smaller than on original design. Some planes have extremely large number of variants, because extremely fortunate initial design accepts lot of additions and them constantly enhance characteristics.