Increase hotend temperature on Monoprice Ultimate 2
Ok, first of all you need to understand why the limit is given at 250 °C: that's when the hotend design chosen by the Monoprice starts to destroy its PETG liner. To print hotter, you'd need an unlined hotend, which means you'd need a proper all metal hotend.
A proper all metal hodend is a tradeoff, not an upgrade: you sacrifice some ease of printing and risk some clogs in the lower temperature band for being able to print higher temperatures.
When you want to buy an e3D hotend, you should go for a genuine one, as many clones are actually not all metal. Also note, that there are other all-metal hotends around.
If you get an e3D v6, you should make sure to buy the right variant for your machine! Monoprice tells us, that this is a 24 V Machine, so you'll need to get the 24 V Variant! As you also will swap the temperature sensor, you might need to update your firmware according to the e3D installation manual. Since a genuine hotend comes with all the cabling and a new thermosensor, you'll need to rewire the whole machine - and you might need to make new fittings before installing. The standard e3D Thermistor that comes with the hotend is good up to 285 °C, which will be your new ceiling temperature. Take careful note of these two parts that appear in the before assembly notes section of the manual:

The standard V6 is capable of printing up to 285°C, do not exceed these temperatures unless you have replaced the Thermistor cartridge with a PT100, the Aluminium heater block with a Plated copper heater block, and the Brass nozzle for a Plated copper, Hardened steel or Nozzle X.
Firmware modification is not optional it is a mandatory step.