Can i use step-up transformer to transform 110v to 220v for my window AC?
Suppose you did.  The unit is labeled for 2500 watts, which means that is how much power  you must provision. 

Let's allow 3% for transformer losses; now we are at 2575 watts.  An air conditioner is treated as a continuous load, so like a lot of things, you must provision power for 125% of the actual load.  So 2575 watts x 125% = 3219 watts.  

If you've heard of an 80% derate, that's the very same thing we just did (inverted) so we don't need to derate  further.

3219 watts is 26.8 amps.  That's too much for a 20A or 25A breaker.  A 30A breaker is required.  For that,  your circuit requires 10 AWG copper or 8 AWG aluminum wire.  You will need to run this as a new circuit because I guarantee you won't find this in your existing wall.  And don't even think about overloading a 12 AWG or 14 AWG circuit at 26.8 amps!!! 

Or, change the whole circuit to 240V

Turn the breaker in question off and then stick a lamp in every receptacle and see what lost power.  Two questions. 


Imagine you couldn't plug 120V appliances in there anymore. Could you live with that? 
You have to have a 120V receptacle within 6' of every point along a wall. (i.e. within 6' of the a threshold, then every 12' after that). If these receptacles disappeared, would you still have that? 


If both answers are "yes", then you can convert the entire circuit to 240V, and change Every Single Plug to NEMA 6-15.  Obviously 120V things won't plug in there anymore, but that's a good thing! That prevents you from setting your 120V appliances on fire.  



A few appliances are capable of working on multiple voltages. Look at their power supply rating to see if they say something like "100-240V" or "90-264V" (that's 100-240V +-10%). For those appliances,  you'll need to change the plug to NEMA 6-15P. Are you daring to think the thought? Don't do it! DON'T leave a NEMA 5 receptacle on a 240V circuit merely so you can plug in dual-voltage devices.  That's how you burn houses down.  The right way is install the proper NEMA 6 in the wall, then change the appliance cord.   Worst case, use a cheater cable. Because then, it's obvious that it's a cheater cable.