In your `.tmux.conf`
```
# remap prefix from 'C-b' to 'C-a'
unbind C-b
set-option -g prefix C-a
bind-key C-a send-prefix
```
or from the command line
```bash
tmux unbind C-b
tmux set-option -g prefix C-a
tmux bind-key C-a send-prefix
```
The latter is important as, while I'm happy with `C-b` ordinarily, if I am in a tmux on my Thinkpad,
ssh into my HP N40L, and open a tmux there, I need to remap the prefix key, but if I ssh in from
a terminal that is not a tmux, I don't need to remap. I then also
```bash
tmux set -g status-bg blue
```
so that the tmux in a tmux is clearly distinguishable from the outer tmux. (Note that I'm not running
a tmux in a tmux on the same machine: one tmux is local, the other remote.)