The guide below uses the 777 permission merely as an example. In most circumstances, using 777 is excessive and is a sign of a problem with your server's configuration - in this case we'd advise you to contact your hosting provider.
In general, we'd advise you to try setting the permissions of the cache/ directory to 755 and the files inside it - to 644. If you absolutely must use higher permissions, try using 777 for the cache/ directory and 666 for the files in it. Avoid setting the permissions of regular files to 777.
To change permissions, you can use a FTP program like FileZilla or you can even use File Manager from your cPanel.
We would recommend you use FileZilla to do this task. Changing the permissions is very easy.
First, make sure that you have an FTP account. If you don't have one - create an FTP account from cPanel.
Now, login via FTP using FileZilla and navigate to public_html/wp-content/themes/themename/
Right click on the cache/ folder (in the root directory of the theme) and select Permissions from the context menu.. Type 777 in the "Numeric Value" field and click on Recurse into subdirectories. Choose Apply to all files and directories. Finally click on OK.
Here is the official WordPress guide: