Voltage regualtor with 5v output @ 2.5 amps
Sounds like you need a switching converter.

These are very efficient voltage regulators that use the charging and discharging of an inductor and a capacitor to regulate voltage.

You could make one yourself, but you'd probably need a custom PCB for it, and I think it would be a better route to just buy one.

2a at amazon: https://www.amazon.ca/VIKINS-Adjustable-Regulator-Converter-Voltmeter/dp/B01CG9QHSG/ref=sr_1_sc_1?ie=UTF8&amp;qid=1460824393&amp;sr=8-1-spell&amp;keywords=swiching+regulator

And a whole host of 3a switchers at ebay, if you don't mind the wait: http://www.ebay.com/sch/i.html?_odkw=switching+regulator&amp;_osacat=0&amp;_from=R40&amp;_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xswitching+regulator+3a.TRS0&amp;_nkw=switching+regulator+3a&amp;_sacat=0

Basically, just search around for a "3A switching regulator".

One more thing: Since you're drawing from a battery, your input voltage probably won't be 6v most of the time, and it might go under 5v that you want to regulate to. What you need then is called a "Buck-Boost switching" converter. This takes any voltage within a range (say, 2-15v), and brings it either up or down to your target voltage.