It depends a bit on what changes you want to make. The starting point is the .per file, which is a plain text file that you can edit with any suitable text editor.
If you want to change the text on the screen, you edit in the changes you want, keeping the data fields between square brackets unchanged (in size, and usually in relative position).
If you’ve added a column to the table, you’ll need to add a new field for it at an appropriate place in the form with an appropriate field tag. You’ll also need to add the tag to the attributes section and identify the table it comes from, and add suitable field attributes.
If you need to add a new table or an extra screen, you have to work a bit harder, but you should be able to follow what’s already there for guidelines. Setting up joins etc requires an example, but the chances are you won’t need that.
Don’t under-estimate the value of generating a default form for a new table and then editing relevant information from that into the existing form.
$ sformbld -d temp-form-name stores extra_table
That gives you a temp-form-name.per file for table extra_table in the stores database.
If you're into (still useful) historical artefacts, you could look up Using INFORMIX-SQL, 2nd Edition by one Jonathan Leffler. It's only available second-hand. It's skewed towards Informix SE or Standard Engine (Informix Online was released the year before the 2nd Edition was published), but most of what it says still applies.
There is documentation available on the IBM web site. Finding it is tricky. Nominally, the easiest starting point is probably http://www.informix.com, which will lead you to a relevant section of the IBM site. The ‘www.’ part matters; omit it and you go to a different, more general (and hence less useful) part of the IBM web site.
However, the referenced site makes it hard to find the documentation. I started at a different point, the IBM Informix 12.10 Knowledge Center. In the 'more information' section, I clicked on Download PDF Books, and from there clicked on links which lead to Publications for Informix 4GL. Once there, I select the 'Individual Publications, by Product and Version' link for Informix 4GL v7.50.xC5, from whence you can download:
Yes, there's no easy way around needing to know that the Informix SQL manuals are bundled with the Informix 4GL manuals, nor that Informix SQL is product separate from the SQL dialect of the Informix server, which might otherwise be termed 'Informix SQL'. Once upon a long time ago, it was correct to refer to INFORMIX-SQL or Informix-SQL as distinct from Informix SQL, but that got changed to suit marketers. It's also very subtle.