Generally speaking, you want to choose Preferences > Package Settings > SFTP > Key Bindings from the menu. This will open up a window with a split in it; the left part is the default key bindings applied by the package, and on the right is your user specific key bindings (or, if you haven’t created any yet, this will be a new empty file).
What you do is find the binding you want to modify in the left, copy it to the pane on the right, and then change the key as appropriate.
For example, on Linux, the binding in the left is like so:
{ "keys": ["ctrl+alt+r","ctrl+alt+b"], "command": "sftp_browse_server" },
So, you’d copy that over to the right hand pane and change the keys part to be only a single key. Note however that keys must always be a list, though it can be a list with just one thing in it:
{ "keys": ["ctrl+alt+shift+b"], "command": "sftp_browse_server" },
Key binding files are JSON and have a specific format; look to the left hand side of the window for examples of this, but in general make sure that the file starts with [, ends with ] and that all bindings are separated by commas.
Sublime’s JSON format is lenient; it allows comments and for trailing comments on the final item (if you’re familiar with JSON that may catch you unaware).
More information in general can be found in the official documentation on key bindings and in the following videos.