Two ways to improve your FTP - Functional Threshold Power.

There is the "obvious" way, and then there is the "better" way.

The obvious way is to work on your FTP. Combinations of (mostly) long aerobic rides and (occasional) intervals can drag FTP slowly upwards. This is basically base miles for people who understand that approach. Base miles are the foundation of a traditional approach to training and FTP is the foundation of cycling performance (as I wrote previously).

However, unlike the physical foundation of a building, FTP is a dynamic response to the loads put on a body by cycling. If the demands of riding require a bigger base, a bigger base will be the result. This means that you can pay attention to other aspects of power production (other energy systems in the body) and drag the FTP upwards surreptitiously.
In the entry about measuring FTP I mentioned MAP. Maximal Aerobic Power is roughly equivalent to VO2max power. While neither VO2 nor FTP are predictive of race results, an increase in either will enable you to ride faster. MAP efforts - circa 5 minute efforts that are neither purely aerobic nor anaerobic - are really good for competitive cyclists to include in their training. By lifting your MAP, you will pull your FTP along too (the reverse is not as true).
Hence, the better way to achieve a higher FTP is via working on MAP. MAP efforts lift anaerobic capacity, aerobic capacity and aerobic threshold all at once.