Copernicus' CDS Climate database makes it impossibly difficult to get the data. GRIB files are a nightmare to read into MATLAB on MacOS. NetCDF files can be read into MATLAB using built in command NCREAD, but you need to know the variable names, which Copernicus inexplicably leaves out of the write up they provide with each file you download. Google at the time of writing performs pitifully trying to find them, so here they are for your benefit:

https://confluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation#ERA5:datadocumentation-Dataformat

Look for tables 1-13 at the bottom, and use the short names.

Then you will need to know what each axis is. The last one, the third, is time: one per hour.

The other two are space: a  lat/lon grid '0.25/0.25' deg resolution. I believe first one is longitude, the second is latitude, since the first is twice as long as the second.

More here on the grid: https://confluence.ecmwf.int/display/CKB/ERA5%3A+What+is+the+spatial+reference

One more thing: downloading 3.5 months of a single variable proved too large for MATLAB, so you may need to break it into smaller pieces. 

Enjoy!