My disk is full
Have you received an error stating that the disk is full? If you have not already done so, verify that your disk is full by running the following commands:
df -h df -ih
The first command will show if your disk is full, while the second will let you verify whether or not your filesystem has inodes remaining. If you find that either is full, then you will either need to find and remove any unneeded data or you will need to resize your disk.
To find the largest directories on your disk, you can run the command:
du -a / | sort -n -r | head -n 5
The 'head -n 5' part tells the command how many entries to return. You can change that number to whatever you like if 5 is not enough.
To upgrade the SSD on your Linode, you would have to upgrade your Linode plan, as it is not possible to upgrade only the SSD. Alternatively, you could use a Block Storage volume to add a second drive and then move your data to it. This link explains what Block Storage is, along with it's associated pricing. This guide explains how to use it.