I would create a variable and add an action to a box. Then create two identical text boxes, one with bold text and one without. Then add a conditional show condition to hide when the variable contains a specific value.
Create vShow variable
Create two text boxes
Format one box to have bold text and the other to have a regular text
Add Set Variable action to one box to set vShow to 1 and a conditional show expression that would be vShow = 0
Add Set Variable action to second box to set vShow to 0 and a conditional show expression that would be vShow = 1
This way when a user clicks on one box it will set variable to 1 and hide that box and reveal the box with bold text. When user clicks on box with bold text, it will set the variable to 0, hide the box with bold text and reveal the box with normal text.