Two ways i think:
1, you can cheat and press F12 to bring up the console and type 'event 23100' this will give you an instant prolitarian bloodless revolution.
2, You'll have to cause a revolution with socialist or communist revolters. It'll take a while to make this possible. Lower class workers often become socialist later on and communist when the socialists are unhappy. Allowing socialist unions in reforms may help. Once you have some socialist pops you need to target them specifically to make them unhappy. So take a look at the pops and try and force a party into a power that they will hate (which you can't do in a democracy), raise lower class taxes to max and inact reforms that will make them angry. Try and make them lose their jobs as well by closing factories with socialist/communist craftsmen or RGOs with farmers and labourers. When the revolution begins, check to see what revolters are coming up. Kill any conservative, reactionary, radical or liberal revolters with your own army and let any socialist/communist revolters take your capital and capture land.
This is just a theory, i've never done this myself, it may be a bit of effort but you'll get a fancy new commie flag for your troubles.
It's also worth noting that early on your lower classes will be conservative due to lower pluracy, if you simply make conservatives unhappy they will turn into reactionaries who will want to install a monarchy. Increasing pluracy through cultural techs and liberal parties etc. will probably help them to become socialist. The classes that naturally turn socialist are craftsmen, laborers, soldiers and if you have equity as your national value clergymen but not farmers.