Using Linux Mint, I have a 1 TB disk in 4 partitions. I haven't been able to use the largest partition.

I am just learning Linux and forgive an old newbie.I haven't been able to write to the sba4 partition. It is owned by root and I don't have permissions. How do I change the ownership to my user. I am the only user on the system, but I don't understand the procedure to chown from root to user. I have found several mentions of changing ownership from user to root, but not the reverse.Needless to say to have a partition of 800 gigs that is unusable is a rather large waste.
Thanks for all help.
ArchyD