Where is the oil pump located?

It is inside the engine mounted to the rear main bearing cap. To replace it you need to drain the oil, then remove the oil pan, then you can access the pump to remove it. However, if you are replacing the pump due to a low-pressure issue it isn't likely that replacing it will actually help. Low oil pressure is more related to worn camshaft, main and connecting rod bearings.
In that case you will want to rebuild the engine to bring it back into specification as a replacement pump won't change the pressures if it's due to worn bearings.

https://www.2carpros.com/articles/high-or-low-engine-oil-pressure