There two different ways to extend a network......one of them is to connect devices using a WiFi connection......and the other much better solution is to use a permanent wired Ethernet cable connection between the WiFi routers.
Depending on the model of AirPort Express that you have, it may or may not be possible to use the preferred wired option.
So, the first question here would be which model of the AirPort Express do you have? Does it have two Ethernet ports? Does it have only one Ethernet port?
If it has only one Ethernet port, look on the side of the Express for the model number. It is hard to see......gray lettering on a white background. The model number starts with an "A" followed by 4 numbers. Example.......A1264.
If you do not plan to connect the second "extending" AirPort using a permanent wired Ethernet cable connection, then you will need another AirPort Express to be able to "extend" the network. The reason for this is that Apple's "extend" function is proprietary......designed to only work with other Apple devices.
If you plan to use a wired connection between the devices, it is still advisable to add another Apple AirPort device so that the products will complement each other and you will able to administer both devices with the same AirPort Utility application.
While Apple has discontinued the AirPort line, you can still find new, refurbished and used AirPort products on Ebay.
Finally, depending on the age of your AirPort Express, your best option might be to retire the device and replace it with one of the newer "mesh" wireless systems that will provide wide coverage. Apple currently offers mesh products from 3 different manufacturers.