Redmine

See this.
Not ease task. The bigger is the leap the bigger is the difficulty of doing upgrade.
Proposed leaps: 1.0.4 -> 1.3.x -> 1.4.x -> 2.0.x ->latest
Check plugins. Some of them have different versions prior 2.0.x. Collect appropriate versions of the plugins you use first and then do upgrade.
From 1.4.0, Redmine used bundler. Yours 1.0.4 does not.
From 2.0.0 use mysql2 instead of mysql gem
What is behind apache in your configuration? Mongrel? You have to change it with thin or puma.
Ruby version? Now I use 1.9.3.
Rails? You have to upgrade rails to latest, and you have to check which rails versions are used in intermediate steps proposed above.

See here I wrote some instructions there which are out of date but applicable in your case.

Thanks Ivan, looks like it will be allot of fun to achieve the upgrade, was hoping it would be easier haha :)

I want to upgrade the redmine of my organization 1.0.4 to 2.6.0
Which steps do I have to follow?

Regards;