70°C = 158°F and does not sound normal.  I don't believe outside of the case should be warm-hot.

7 Chrome tabs, OneDrive, and a YouTube video hardly sounds like a load and certainly not for an i5-8500.  It might wind up being your best bet to use the Contact Support link, as that's the best way to go for in warranty items.  It lets you call, chat, or email.

Meanwhile, you still might get ideas from other Community Members as well.

It might get asked - Do you have the latest BIOS for your 7070?  This is just in case your current BIOS is under-utilizing the fans.

I don't know if Matlab would warm up your system or not.  It looks your system exceeds minimum requirements, especially if you have 8GB ram and not 4GB.

For now, least 70°C is well below the Junction Temp. of 100°C.

I am returning the computer since it does not work well with Matlab. 

I don't know if there is a setting, but it is slower than i5-8250u with higher operating temp.

This pc will only work for light load work like ms office....

Don't be fooled by full i5-8500 (not i5-8500T) in this box. 

I'm almost glad you're returning it.  Some smaller PC's, and especially micro's, just don't seem to have the same amount of as bigger PC's.  The smaller the PC, the smaller the PSU.  Hopefully you get something bigger.

One complaint I heard about micro's was it didn't seem to have enough power for the USB's, so even using a printer was difficult as compared to something larger.