Oracle Error: the sga requires more space than was allocated for it (Oracle SQL 12c running on Windows 10)

My installation has previously been working fine. I was trying to set up one of my tables to use the In-memory option available in 12c. I was following this guide: https://blogs.oracle.com/In-Memory/entry/getting_started_with_oracle_database
I wanted to allocate 4Gigs of system RAM to be used as in-memory table storage space for a table I would specify later. To do this I ran these commands using the guide as a reference:

This one to allocate 4G of RAM space:

ALTER SYSTEM SET inmemory_size = 4G scope=spfile;


And this one, which is what caused the problem:

ALTER SYSTEM SET sga_target = 75G scope=spfile;


The guide said "By default, the installer set the SGA_TARGET to 71G, " which is where I came up with 71G + 4G = 75G.

Now of course I get the error when trying to connect to the DB:


  ORA-27101: shared memory realm does not exist 
  
  the sga requires more space than was allocated for it


Since I can't connect to the database because it won't start, how would I fix the invalid setting? Is there some sort of "undo"?

Edit:
No, there is no backup of the spfile or pfile that I can use to generate the spfile. I have manually changed the sga_target to small values that should be fine but still get the error no matter what. I also tried creating a new, additional database. When I did that, the other database started working for some reason. It just looks like the new one though and is missing new tables I had previously created.
Before you start make a copy of the files you change (the spfile) 
Check where your spfile is located, on Linux this is $ORACLE_HOME/dbs/spfile${ORACLE_SID].ora. If this does not exist check the content of $ORACLE_HOME/dbs/init${ORACLE_SID].ora. There the spfile parameter may be set which defines the path of the spfile. For Windows look in the %ORACLE_HOME%\database directory instead of.
Open sqlplus and create a pfile from the spfile create pfile='&lt;auxiliarypfile&gt;' from spfile='&lt;spfilepath&gt;'. Do not overwite an existing pfile but write the file in an arbitrary directory.
Edit the pfile &lt;auxiliarypfile&gt; created in step 3 with a text editor like notepad.
Open sqlplus and overwrite the spfile by a new created spfile create spfile='&lt;spfilepath&gt;' from pfile='&lt;auxiliarypfile&gt;'