On your Symplectic profile, click on "Publications" and find the relevant title in the "My publications" list. Click on the title of the publication to go to the publication details page. Under the "full text" heading click on “Deposit” to go to the deposit page.
You can then select the file to upload. Please upload the "accepted version", label it as such from the drop down menu, and press the upload button. You’ll then need to tick the box to agree to the repository licence terms and conditions and finally press the “Deposit my publication” button to complete the upload.