How do we increse the range miles of the FM transmitter?

there are some toy walkie talkie's available in the market. bu they have a range of only tw0 to three meters. IS it possible to modify them to create a good range upto 5 miles or more. Whats is required for this.? and what blocks of the Transmitter and receiver need to be modified?

Rules and regulations for radio transmission vary widely from country to country but generally in each country there’s some sort of Federal Communications Commission (FCC) says that all transmitters used in service must be FCC certificated, or type accepted. That means that the FCC has to approve the design of the transmitter. If it approves the design, it provides a certificate on the FM that states that it is type accepted. FCC rules states that you are not allowed to make any modifications to your transmitter that will cause it to transmit with illegal power. Consequently, the FCC has authority to impose fines on people who violate the rules. The fines and penalties can be severe, so better to avoid any modification even to such radio toys.
On the other hand, it’s not simple to modify a transmitter without using special RF measurement equipment. In my opinion it's not worth the effort.

We intend to built or modify the FM transmitter for our educational purpose. We are students. And our institution is the support that helps us in doing such practices. Like, one of the students made a rocket. He was provided a site to launch his rocket with the supervision. So it's not our problem.
could you please mention what is that special RF equipment?

For building a FM transmitter you need a license from a government authority. If your school officials do not know it, the responsibles will be punished.

If you want to expand the range of "small walkie-talkies", the best way is to build a pair of Yagi antennas for the particular frequency, and try to couple the whip monopole to the antenna for the best results.
The next step is to climb on hills; the VHF/UHF signals best propagate over a free space.

In 1965 I used a 5 mW transmitter and a superregenerative receiver with a 10-element Yagi to communicate at 432 MHz over 205 km the described way. So I know it works.