If you're asking about the By Roll20 sheets, the answer is usually no - they maintain control over the development of these sheets. If it's any other sheet, then the code can be found on the roll20 github, and modified as you see fit for your own use. There are old versions of some of the By Roll20 sheets on the Github, which you can modify if you don''t mind using an older sheet. For most systems, it probably doesn't make much difference. But you can't do this with the D&D 5e sheet - that one isn't public any more. You can use one of the sheets thats based off an older version of it, from when it was public. I think Darker Dungeons is one of those. But if you want specifically to use the 5e sheet and enjoy any updates and maintenace that is made with it, the answer to your question is a very definite no.