Upgrading SSD for DELL inspiron 15 7577

Hello,

That model supports both sata and NVMe  sata limit is 256GB and NVMe limit is 512GB however those limits mean Dell didn't test larger drives with that particular system..You have several choices Crucial , Samsung , Corsair , Sandisk , Kingston just look out you purchase an ssd with the correct key or it won't fit in your M2 socket .

Thank you for your help and sorry for being late to response
Another questions
Will  It be any side effects on the laptop?
Which size and type you would advice me to purchase?
and how can I know the correct key? 

Are you making use of your separate sata hard drive with lots of space? The ssd is the C drive and is primarily for speeding up system files and should not be used for storage or downloads. Unfortunately Windows sets the C drive as the drive for storage and ignores the big sata hard drive. You have to change this in Settings, System, Storage and make the sata drive D your main storage drive for all downloads and program installations. Changing the ssd drive with all your system files is not recommended.

To check your drives, right click on the Start button and choose Disk Management.

Thank you for responding, 
So, you don't advise upgrading the SSD to a larger space?
But I really need to find a solution to the inadequate space problem, any suggestions?

Regarding your question, Yes the windows is installed on the C drive (the 128 SSD) and it kept on warning me about the inadequate space from time to time. I use programs that are demanded to be on the C drive for better performance, that is why your instructions would help me just temporarily, I wanna solve the problem roots, not just cleanup, 
If you could provide any recommendations concerning the compatible types for my device?
How could I transfer the original windows to the new SSD If it is upgradable? Best regards