How to upgrade to more proxies?

You can re-customize your subscription anytime during the month. You will be credited for any unused bandwidth/time in your current plan. 

Since you can only have 1 subscription at a time, you should upgrade to the final number of proxies you'd like to have in your plan. For example, if you have 100 proxies and upgrade to 250 proxies, you will keep the 100 previous proxies and receive 150 more proxies.