Hi @ibrahimshoki
The laptop has 1 x M.2 SSD slot and 1 x 2.5" sata HDD connector so you can upgrade the SSD and replace the HDD with an SSD with a higher capacity storage version but you cannot add an extra SSD.
You can replace the existing M.2 SSD with an M.2 2280 PCIe NVME SSD having a capacity of up to 2TB (2000GB) and the existing sata HDD with a 2.5" 7mm sata SSD having a capacity of up to 4TB (4000GB)
Here's the [https://dl.dell.com/content/manual34342314-dell-precision-7740-service-manual.pdf?language=en-us|owner manual] that may help. Go to p.41 and p.467 to view the procedure to remove/replace the SSD and the HDD respectively.