Depends on your database size and complexity you can either export/import data, or you can migrate whole database. Both ways seems to be pretty time consuming :)

In our small business we also use odoo9c and we are using contacts and project management apps, so not a big deal. To run multiple versions (and instances) I use python virtualenv, which I prepare separately for each odoo version I plan to deploy. (I also run virtualenv for production).

I decided to upgrade to odoo12 as it has significant features improvement - specially the responsive mobile version - which I like.

I followed oca/openupgrade \tutorial\\&nbsp\;and\ use\&nbsp\;\automated migration tool. Migrating to odoo10 was relative simple as it is build on python2.7 (same as odoo9). 

But migrating to odoo11 is more complex as it is build on python3, so there is lots of changes. At the end I was not able to fix all the errors after the migration to v11 (I'm not too experienced in fixing db models etc), however the server runs. But I was not able to migrate to odoo12 as migration script does not support it (yet).

As our database is not very big and we are using only few apps I finally decided to install brand new database and import the data from our old odoo9c. This doesn't seems to be easiest approach, but considering the time I spent on migration of whole db and fixing subsequent errors I think it is the best approach in situation like this (small business, small data sets).

Hope this helps a bit.