Hi and Welcome!
HDDs are often the parts of a laptop that can be replaced most easily.
It is a good idea to check whether a maintenance manual exists (such exist with Dell, HP/Compaq and IBM/Lenovo).
For other brands, googleing for “myLaptop dismantling” will often give excellent results.
Since you want your HDD replaced by something bigger, you should also find out if your notebook supports larger HDDs than 120 GB.