First, check if using the bevel modifier by itself works. Make sure your scale is applied as well.
If the bevel does work, you will need to create a vertex group with all the top faces of the text selected. You can then use that vertex group as a mask for the bevel modifier.
If the bevel modifier doesn't work, let me know and we can sort something out.
EDIT:
I did some experimentation, and it turns out that the text object in Blender has a bevel option! Simply just edit the bevel settings the way you want, then convert the text to mesh. You can then delete the bottom section of the text, re-extrude downwards, and voila!

The screenshot above is from Blender 2.92, and I see you're using 2.82. Most of the options should still be there, but if they aren't...
Then you'll have to retopologize your text and then apply a bevel modifier on that.