Welcome to the Community. I suggest you uninstall and reinstall the touchpad drivers. To uninstall, click on start, right click on computer, click on properties, click on device manager, click on the symbol next to pointing devices, right click on Alps touchpad and click on uninstall, click ok to confirm. Once its removed use the below link to install it.
Restart your computer after installing the driver, and then click on start, control panel, mouse, wait for the Dell Touchpad image to appear and then click on it, and you should be able to adjust the sensitivity.
Hope this helps.