Is it possible to convert string to uppercase? Is it possible to convert a simple string ("Hello World!") to uppercase ("HELLO WORLD!")?
The upper filter is what you are looking for:

{% set var = 'Hello World' %}

{# Print in UPPERCASE #}
{{ var|upper }} {# 'HELLO WORLD' #}


Besides upper there are also some other filters available to modify the capitalization style of your strings. Of course there is the lowercase equivalent lower and additionally there are some Craft exclusive filters available.

&lt;ul&gt;
  {# Set test strings #}
  {% set strLower = 'this is a test string' %}
  {% set strUpper = 'THIS IS A TEST STRING' %}
  {% set strMixed = 'this Is a TEST stRing' %}

  {# Apply Twig filters #}
  &lt;li&gt;upper: {{ strLower|upper }}&lt;/li&gt; {# 'THIS IS A TEST STRING' #}
  &lt;li&gt;lower: {{ strUpper|lower }}&lt;/li&gt; {# 'this is a test string' #}

  {# Apply filters provided by Craft #}
  &lt;li&gt;ucwords: {{ strLower|ucwords }}&lt;/li&gt; {# 'This Is A Test String' #}
  &lt;li&gt;ucfirst: {{ strLower|ucfirst }}&lt;/li&gt; {# 'This is a test string' #}
  &lt;li&gt;lcfirst: {{ strUpper|lcfirst }}&lt;/li&gt; {# 'tHIS IS A TEST STRING' #}      

  {# Combine filters for predictable application #}
  &lt;li&gt;ucwords: {{ strMixed|lower|ucwords }}&lt;/li&gt; {# 'This Is A Test String' #}
  &lt;li&gt;ucfirst: {{ strMixed|lower|ucfirst }}&lt;/li&gt; {# 'This is a test string' #}
  &lt;li&gt;lcfirst: {{ strMixed|upper|lcfirst }}&lt;/li&gt; {# 'tHIS IS A TEST STRING' #}

  {# Examples for possible use cases #}
  &lt;li&gt;PascalCase: {{ strMixed|lower|ucwords|replace(' ', '') }}&lt;/li&gt; {# 'ThisIsATestString' #}
  &lt;li&gt;camelCase: {{ strMixed|lower|ucwords|lcfirst|replace(' ', '') }}&lt;/li&gt; {# 'thisIsATestString' #}
&lt;/ul&gt;