Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
That Keyboard should have a Micro USB port. So you need to buy a Micro USB cable and adapter and use it to charge. Or just buy the Micro USB cable and charge from your computer.
You can also use a phone charger except Apple iPad/iPhone charger to charge. Chances are, you may have the cable there as these are standard cable for most of all android phones.
If this is a Keyboard variant without the USB port, then there are one or two small batteries on it you can replace. On the top left side of the keyboard, you will see a cover that you can open. Open it and you will find two battery cells. You can replace them. See this video --> https://www.youtube.com/watch?v=c58FjnQffhw
You need to buy the CR2032 3V battery cells to replace the existing ones. See the video from 35 seconds onwards --> http://https//www.youtube.com/watch?v=c58FjnQffhw
Logitech has released several variants of this keyboard. Since you didn't give the exact model number, it is hard to know which model.
If it has inbuilt rechargeable battery, then you will see a small USB port on it (on the sides) where you can charge using the micro-USB mobile phone charger or cable. For this model, you don't have to replace battery.
If you don't see the USB port on it, then it has removable and non rechargeable battery. So you have to replace them. You will see the battery tab at top left which you can pull out to get the batteries.