Double Double Quotes

You don't need the first ampersand. Your single quote doesn't need to be in a string on its own

   DoCmd.OpenForm "UpdateCustomerF", , , "customerID='" & customerNum & "'"

I prefer using Double Double Quotes as detailed in the video Sami linked you to.

Out of curiosity:
   What Data Type is CustomerID?
   Why have you named the fields you are comparing differently?

Quick note:
   I like to prefix all my Buttons with btn so I know at a glance I am working with a Button.
   Also:
      cbo: Combo Box
      lst: List Box
      lbl: Label (is referred to in code)

Hi Kevin.  customerID is short text because a customerID has alpha and numbers.  
I named the fields differently because I didn't know any better.  Now I am quickly learning :)
I like your naming convention, and I see its advantages.  
But where did you learn all the ins-and-outs of the quote dilemma?  There's gotta be a better way than trial and error?  There has to be a published protocol, right?
Many thanks for your help.  All the best to you.

I prefer Double Doubt Quotes except in my case in can be closer to Double GetinTroubleWiththeDouble Quote.  I will get it eventually but still.

Is there a free reference (online or pdf) that explains all the VBA commands and all the options that go with them?  There are so many commands, clauses, filters, options, that there has to be a reference somewhere.  Thanks for all the help.

https://learn.microsoft.com/en-us/office/vba/api/overview/access

Thanks for the reference Alex.