Select the desired borders first and edit them.
Experiencing
Feature Synap Editor A B C D Importing DOC, DOCX, HWP and ODT documents ○ - - - - Supporting multilevel list ○ - - - - Supporting custom bulleted list ○ - - - - Inserting layer ○ △ - - - Editing and moving layer ○ - △ - - Supporting drag&drop for contents ○ △ △ △ △ Increasing/decreasing the font size by a level ○ - - - - Providing the ruler feature ○ ○ - - - Table style ○ ○ △ - - Width/height autofit ○ ○ ○ - - Selecting borders ○ - - - - Copying/pasting cell contents ○ ○ - △ △ Copying/pasting cell style ○ - - ○ ○