Yes, you may replace your hard drive with an SSD. You will notice a distinct performance increase.  Follow the removal and installation instructions in the Dell manual. 
Dear Mr. Jesse L. & All Community Members, With reference to the post & the accepted community solution written therein - - [URL]#M49515 IS THIS TRUE?  THE CRUCIAL WEBSITE CLEARLY STATES THAT IT SUPPORT DELL INSPIRON 15 3521. I have a Dell Inspiron 15 - 3521 Can I install an SSD in it ? - Is this Dell laptop model compatible for installation of SSDs ? I am planning to install Crucial SSD - The Crucial SSD Website says it is compatible with Both Inspiron 15-3521 & Inspiron 3521. LINK1 - [URL] LINK 2 - [URL] With Thanks & Best Regards, Mr. Vikas Kulkarni.  
SSD FOR DELL INSPIRON 15 3521 & inspiron 3521