Ok, I'm back at my computer, so let me scrap my old post and give you better directions.  First, turn on spacemaster mode from the main taskbar.  Then go into the Systems Information screen by either clicking the Sun icon at the top of the System Map(F3) screen or pressing F9.  This brings up a list of all the bodies in the system, organized in tabs by the star they are orbiting(Gliese 762-A, Gliese 762-B and so on).  At the bottom right there will be a number of buttons in a box labeled Spacemaster Functions.  Click on the planet you want to adjust the minerals on, then click the button that says Specify Minerals.  A box will pop up that lists all of the minerals, the size of the deposit, and the accessibility.  You can then change them however you want, just type in a new number and select the accessibility from the drop down menu.  Just make sure to click Save Changes before exiting or selecting a different system body.  I hope this helps.

I wanted to try a conventional start but keep having issues with mineral supply.

Is there a way to change the amount of minerals on a planet after it's been surveyed and the geological survey has been completed?  I'd like to change the quantity of minerals available to be mined.