I am guessing you are using a Hole Table?
If you want to just do this one time and want to do it as quickly as possible, you could simply select the cell, and backspace the -6H part out of the "Dimension Text" that appears in the PropertyManager.
If, however, you want to fix this problem for good, you will need to locate your hole callout template file. Go to Options (under the Tools menu), and while on the System Options tab click File Locations. Use the Drop-down menu to find Hole Callout Format File.
I run SolidWorks 2015, so my default location is: C:\Program Files\SOLIDWORKS Corp\SOLIDWORKS\lang\english
Verify what your file location is though.
Navigate to this folder in windows, and open the file called calloutformat.txt
Scroll down to the section for ANSI Metric Tapped Holes. Remove the -6H part of the callout wherever you see fit. Save your file, and it should no longer add the 6H to your hole callouts. If for some reason it doesn't update in SolidWorks, you can try one of the following:
Recreate your hole table
Hope this helps!