Most probably, the RAM is being used by system programs and you should not clear it.
But if you want to do it, you can go to 'task manager' (right click on task bar and select 'task manager').

Go to 'applications' tab and see if there are any programs that are loaded in memory even though you are not using them. You can select and close them to recover teh RAM.

Do the same for 'Services' tab in this window.

NOTE: You should be well aware of the programs you closing. If you don't know what you are doing, it is best to avoid it.