I suggest you start by reading this article:

Filing T4 Slips Electronically (sage.com)

As for adjusting T4 boxes and amounts - it is POSSIBLE to edit the .xml file (the electronic T4 file) manually, but there are ABSOLUTELY NO SAFETIES if you choose this option.  I always recommend trying to fix things within Sage so that the .xml file is right the first time.

Here's an article that may help with that:

Add or Modify an Employee's T4 or RL-1 Report Information (sage.com)

Most changes to T4s should be done in the payroll module with an adjustment paycheque entry.  If you edit the xml, the changes will not appear on the employee or company copies of the T4s.  If you are not able to do this, then you can efile through Sage firts, then amend the T4s through CRAs Webforms.

https://www.canada.ca/en/revenue-agency/services/e-services/filing-information-returns-electronically-t4-t5-other-types-returns-overview/filing-information-returns-electronically-t4-t5-other-types-returns-file/filing-web-forms.html